A Defect Study and Classification of Brown Diamonds with Deformation

minerals

Article A Defect Study and Classiﬁcation of Brown Diamonds with Deformation-Related Color

Thomas Hainschwang 1,*, Franck Notari 2 and Gianna Pamies 1 1 GGTL Laboratories, Gnetsch 42, 9496 Balzers, Liechtenstein; [email protected] 2 GGTL Laboratories, 2 bis route des Jeunes, 1227 Geneva, Switzerland; [email protected] * Correspondence: [email protected]; Tel.: +423-262-24-64

Received: 3 September 2020; Accepted: 7 October 2020; Published: 12 October 2020

Abstract: For this study, the properties of a large sample of various types of brown diamonds with a deformation-related (referred to as “DR” in this work) color were studied to properly characterize and classify such diamonds, and to compare them to pink to purple to red diamonds. The acquisition of low temperature NIR spectra for a large range of brown diamonds and photoexcitation studies combined with various treatment experiments have opened new windows into certain defect characteristics of brown diamonds, such as the amber centers and naturally occurring H1b and H1c centers. It was determined that the amber centers (referred to as “AC” in this work) exhibit rather variable behaviors to annealing and photoexcitation; the annealing temperature of these defects were determined to 1 range from 1150 to >1850 ◦C and it was found that the 4063 cm− AC was the precursor defect of many other ACs. It is suggested that the amber centers in diamonds that contain at least some C centers are essentially identical to the ones seen in diamonds without C centers, but that they likely have a negative charge. The study of the naturally occurring H1b and H1c link them to the amber 1 centers, specifically to the one at 4063 cm− . Annealing experiments have shown that the H1b and 1 H1c defects and the 4063 cm− AC were in line with each other. The obvious links between these defects points towards our suggestion that the H1b and H1c defects are standalone defects that consist of multiple vacancies and nitrogen and that they are—in the case of brown diamonds—a side product of the AC formation. A new classification of DR brown diamonds was elaborated that separates the diamonds in six different classes, depending on type and AC. This classification had been completed recently with the classification of brown diamonds with a non-deformation-related color (referred to as “NDR”), giving a total of 11 classes of brown diamonds.

Keywords: natural diamonds; defect characterization; optical properties; plastic deformation; Vacancies

1. Introduction Brown diamonds (Figure1) make up a considerable percentage of the diamonds produced worldwide, particularly from certain deposits like the Argyle mine in Australia. They represent a large group with variable properties and were—in combination with this work—classified recently [1]. With the color “brown” not being a spectral color, but a mix of different colors (a composite color), it is often quite difficult to define; brown is produced by mixing red and green, orange and blue, purple and yellow, or simply by darkening orange or red, hence mixing red or orange with black. In diamonds, the brown color often exhibits modifying hues, such as yellow, orange, green, and pink. The brown color in most diamonds is associated with post-growth plastic deformation and the defect responsible for this color of such plastically deformed diamonds was identified as large spherical aggregates consisting of 40 to 60 vacancies, so-called vacancy clusters [2–5]. Not all brown diamonds are plastically deformed, though, as there are particularly CO2 and “Pseudo CO2” diamonds [6] as well as hydrogen-rich diamonds [7] that owe their brown color to other defects.

Minerals 2020, 10, 903; doi:10.3390/min10100903 www.mdpi.com/journal/minerals Minerals 2020, 10, 903 2 of 38

Diamonds of all the different types can appear brown, and practically all colored diamonds can show brown as a modifying color. A classification based on a range of criteria has been established in 2003 [6]; this classification separated brown diamonds into two broad categories, one including diamonds not exhibiting an “amber center” infrared absorption and one including all diamonds exhibiting an “amber center” (referred to as “AC” in this study). This approach defined a total of nine classes, three with AC and nine without AC; it was found that out of the roughly 900 diamond samples tested, about 95% exhibited an AC. The ACs are absorptions caused by electronic transitions—hence not vibrational 1 absorptions—which exhibit their ZPLs (zero phonon lines) in the range between 3462 and 4165 cm− 1 at room temperature and with structures that extend to approximately 10,500 cm− [8]. Since they are electronic transitions, they react strongly to temperature and the absorptions become distinctly narrower at a low temperature, by which means it was demonstrated that the ACs consist of a series of ZPLs [8]. Spectroscopically speaking, the brown color in diamonds is caused by an absorption continuum gradually increasing from the NIR to the UV; this continuum is shallow and overlaid only by minor absorptions—if any—such as N3 (ZPL at 415.2 nm), H3 (ZPL at 503.2 nm), and the 560 nm band [6]. The brown color in diamonds can be induced by treatment, although generally such a result is undesired and accidental, since it does not increase the value and/or desirability of the treated diamond; exceptions are orange brown diamonds, which are colors produced purposely by treatments. Treatments resulting in a brown color include irradiation by most commonly electrons or neutrons and irradiation followed by annealing [9], as well as HPHT (high pressure, high temperature) treatment [10]. The vacancy cluster defects responsible for the brown color in diamonds are very stable and anneal out only at temperatures above 1800 ◦C, with distinct variation between the different diamond types; the defect is more stable in Type II diamonds and anneals out efficiently (i.e., in a few minutes) only at temperatures of 2300 ◦C or more. As a consequence, the only treatment to eliminate the deformation-related brown color of diamonds is a heat treatment above 1800 ◦C, conditions in which diamonds are most efficiently kept from sublimating to CO and then CO2 by applying pressure. Hence, the only way to eliminate the brown color in natural brown diamonds with a plastic deformation-related color is high pressure, high temperature (HPHT) treatment at temperatures typically ranging from 2000 to 2500 ◦C and pressures between 55 and 85 kbar. This treatment is used to decolorize type IIa brown diamonds, to eliminate the brown component in “olive”-colored type IIb diamonds—turning them blue—and to transform the color of type Ia brown diamonds into various shades of yellow, with or without green or orange color components [11]; here, the green color components are always caused by H3 fluorescence [12]. The treatment can also be used to modify the color of brown type Ib diamonds and type Ib diamonds with color modifiers; all type Ib diamonds will turn pure yellow upon HPHT treatment [13]. It has been shown that the HPHT treatment does not eliminate the brown color in diamonds with a color not related to plastic deformation; here, the only thing that will modify the color is the modification of the nitrogen aggregation with increased treatment temperature (i.e., the creation of C centers (single substitutional nitrogen)), and the increase or formation of 480 nm absorption [14]. Synthetic diamonds of brown color have been very uncommon until the ascent of CVD (chemical vapor deposition) synthetic diamonds, which are often brown in their as-grown state due to their very defective structure. Such brown stones are generally HPHT treated to render them near-colorless to colorless, but recently significant quantities of untreated brown CVD synthetic diamonds mixed in parcels of melee-sized natural brown diamonds have been discovered [15]. Before the time of CVD synthetics, the brown color in an HPHT synthetic was, in the authors’ experience, only obtained by irradiating dark yellow to orange yellow stones. The purpose of this paper is a detailed characterization of brown diamonds with a deformation-related color, particularly of the defects that can be detected by various spectroscopic methods, plus a newly proposed classification of such colored diamonds. A series of treatment experiments were used to gain more information on a range of spectral features typical of such brown Minerals 2020, 10, 903 3 of 38 diamonds. The annealing behavior of naturally occurring H1b and H1c and the ACs at temperatures from 150 to 1500 ◦C and 1850 ◦C is presented here for the first time. Another important result from this study is the creation of AC absorption in colorless type Ia diamonds by heavy neutron irradiation followed by annealing at 1000 to 1850 ◦C. The color distribution and strain patterns of the diamonds were analyzed using a Leica M165C Trinocular Microscope, equipped with a Leica DFC450 CCD camera; the color distribution was checked with the diamonds immersed in alcohol or diiodomethane; the strain patterns were analyzed with the stones immersed between crossed polarizing filters. The luminescence of the diamonds was observed under 254 nm shortwave and 365 nm longwave radiation from a model UVP UVSL-26P, 6 Watt UV lamp. Detailed luminescence observations were made using a prototype GGTL Mega-DFI luminescence imaging and spectroscopy system that used high intensity UV in the form of 5 different, more or less broad excitation bands (centered at 350, 320, 280, 260, and 220 nm) as well as 405 nm laser excitation. 1 Infrared spectra of all the samples were recorded with a resolution of 4 cm− , and for some also 1 1 cm− , on a Perkin Elmer Spectrum 100S FTIR spectrometer equipped with a thermoelectrically cooled DTGS detector, using a diffuse reflectance accessory as a beam condenser and a 5 beam condenser. 1 × The spectra were recorded over a range of 8500 to 400 cm− , with 50 to 1000 scans for each diamond. The nitrogen concentration was determined by progressive spectral decomposition via spectral calculations (“progressive decomposition”). The nitrogen concentration was calculated based on the 1 known average absorbance of the intrinsic diamond infrared feature at 1995 cm− , which has been defined by others as 12.3 absorbance units per cm of optical path [16]. All diamond spectra were normalized before the concentration calculation could be reliably conducted. This normalization was performed by spectral calculation, for which the absorbance value of the intrinsic diamond absorption 1 on the y axis at 1995 cm− was measured and then a multiplying factor was applied in order to obtain 1 a value of 12.3 cm− . The spectrum was then multiplied by this factor. The method found to be the most satisfying and precise one was the progressive spectral decomposition in which the individual components (A, B, C, and X center) were subtracted from a given spectrum, using reference spectra of pure signals of the respective centers. The determination of type Ib diamonds and the C center 1 concentration was based on the presence and intensity of the 1344 cm− band and/or its first harmonic 1 1 at 2688 cm− [17], and not on the intensity of the 1130 cm− band because of possible false results from underlying Y center absorption [18]. 1 Near infrared spectra in the range 11,200 to 4000 cm− (900 to 2500 nm) were recorded using a custom-built GGTL NIR spectrometer system using a thermoelectrically cooled InGaAs detector, in a 15 cm integrating sphere using a special high-power NIR light source. Spectra were recorded with 50 1 to 150 scans for each diamond, at a resolution of 4 cm− , at room temperature and with the samples cooled to 77 K. Photoluminescence spectra were recorded on a GGTL Photoluminator RS6 system using 360, 402, 473, 532, and 635 nm laser excitations, and a high-resolution Echelle spectrograph by Catalina Scientific equipped with an Andor Neo CMOS camera, thermoelectrically cooled down to 30 C. The system − ◦ was set up to record spectra in the range of 350 to 1150 nm with an average resolution of 0.06 nm. All photoluminescence spectra were recorded with the diamonds cooled to 77 K by direct immersion in liquid nitrogen. UV-Vis-NIR spectra were recorded on a GGTL D-C 3 spectrometer system using a combined xenon, halogen, and LED light source; a quadruple channel spectrometer with a Czerny–Turner monochromator and a thermoelectrically cooled CCD detector was employed, with an average resolution of 0.3 nm. The spectra were measured with the samples cooled down to about 77 K, placed in an integrating sphere of 15 cm diameter. A series of brown diamonds were treated by the different treatments applied to the diamonds, including annealing only, irradiation by electrons or neutrons, irradiation followed by annealing, as well as HPHT with and without prior irradiation treatment. Some of these treated diamonds have Minerals 2020, 10, 903 4 of 38 been published in earlier articles and some are unpublished; a selection of the most typical and most interesting results will be included in this paper. Irradiation was performed at the electron irradiation facilities of Leoni Studer AG in Däniken, Switzerland, using electrons of variable energies from 1.5 to 10 MeV, with an irradiation time of 2 to 4 h. The exact irradiation doses are generally unknown since (for reasons of treatment cost reduction) the samples were irradiated together with the materials that were passed through the accelerator for the cross-linking treatment. Neutron irradiation was performed in a nuclear reactor in Poland with a dose of 1.8 1017 neutrons/cm2 by mostly fast neutrons of an energy of several MeV (the exact × value being unknown). Annealing and post-irradiation annealing have been realized at our laboratory Minerals 2020, 10, x FOR PEER REVIEW 3 of 37 using a Nabertherm LHT 02/17/P470 oven with a maximum temperature of 1750 ◦C. For the annealing experiments, the diamonds where kept in a reducing atmosphere, to keep the diamonds from burning a newly proposed classification of such colored diamonds. A series of treatment experiments were at conditions above 500 ◦C. Annealing was performed for the diamonds up to a temperature of 1500 ◦C. used to gain more information on a range of spectral features typical of such brown diamonds. The Some samples were treated by HPHT at temperatures ranging from 1850 to 2500 ◦C and pressures annealing behavior of naturally occurring H1b and H1c and the ACs at temperatures from 150 to ranging from 55 to 85 kbar and kept at the maximum temperature for minimum 2 to maximum 30 min. 1500 °C and 1850 °C is presented here for the first time. Another important result from this study is The treatments were performed at the HPHT facilities of the Bakul Institute of Superhard Materials, the creation of AC absorption in colorless type Ia diamonds by heavy neutron irradiation followed Kiev, Ukraine; SedKrist GmbH, Seddiner See, Germany; and Suncrest in Orem, Utah, USA. by annealing at 1000 to 1850 °C.

Figure 1. AA variety variety of differently diﬀerently colored and shaped deformation-related brown diamonds.diamonds.

2. Materials and Methods Besides thethe brown brown diamond diamond samples samples collected collected in the pastin 20the years past by 20 TH years (Thomas by Hainschwang),TH (Thomas approximatelyHainschwang), 30,000approximately brown diamonds 30,000 brown in various diamonds sizes andin various from various sizes and sources from were various screened sources for thiswere study screened and for diff thiserent study groups and of different stones weregroups selected of stones based were on selected their luminescence based on their spectra. luminescence A total ofspectra. 75 gem-quality A total of brown75 gem-quality and 2 colorless brown diamonds and 2 colorless weighing diamonds 0.01 to 61.56weighing ct were 0.01 analyzed to 61.56 inct detailwere foranalyzed this study; in detail 57 samples for this for study; this first57 samples part of thefor articlethis first and part 20 samplesof the article for the and second 20 samples part. Samples for the includesecond allpart. di ffSampleserent diamond include types all different and all di ffdiamonerent categoriesd types and of brown all different diamonds, categories owing theirof brown color todiamonds, deformation-related owing their defects color to (referred deformation-relate to as “DR”) andd defects owing (referred their color to toas non-deformation-related“DR”) and owing their defectscolor to (referrednon-deformation-related to as “NDR”). Only defects diamonds (referred with to aas dominantly “NDR”). Only brown diamonds color were with included, a dominantly all of thembrown facetted color were into included, various shapesall of them with facetted the exception into various of one shapes rough with diamond. the exception The two of one colorless rough diamondsdiamond. The were two included colorless to diamonds represent were the neutron included irradiation to represent treatment the neutron experiments irradiation in thistreatment study. Seeexperiments Table1 for in details this study. on the diamonds.See Table Besides1 for details the diamonds on the diamonds. tested specifically Besides for the this diamonds study, the tested data ofspecifically a large selection for this of study, pink tothe red data to of purple a large diamonds selection analyzed of pink to in red the pastto purple were diamonds reviewed to analyzed gain some in insightthe past into were the reviewed differences to andgain links some between insight pinkinto andthe differences brown diamonds; and links the between details of pink these and diamonds brown arediamonds; not listed the here. details of these diamonds are not listed here. The color distribution and strain patterns of the diamonds were analyzed using a Leica M165C Trinocular Microscope, equipped with a Leica DFC450 CCD camera; the color distribution was checked with the diamonds immersed in alcohol or diiodomethane; the strain patterns were analyzed with the stones immersed between crossed polarizing filters. The luminescence of the diamonds was observed under 254 nm shortwave and 365 nm longwave radiation from a model UVP UVSL-26P, 6 Watt UV lamp. Detailed luminescence observations were made using a prototype GGTL Mega-DFI luminescence imaging and spectroscopy system that used high intensity UV in the form of 5 different, more or less broad excitation bands (centered at 350, 320, 280, 260, and 220 nm) as well as 405 nm laser excitation. Infrared spectra of all the samples were recorded with a resolution of 4 cm−1, and for some also 1 cm−1, on a Perkin Elmer Spectrum 100S FTIR spectrometer equipped with a thermoelectrically cooled DTGS detector, using a diffuse reflectance accessory as a beam condenser and a 5× beam condenser. The spectra were recorded over a range of 8500 to 400 cm−1, with 50 to 1000 scans for each diamond. The nitrogen concentration was determined by progressive spectral decomposition via spectral calculations (“progressive decomposition”). The nitrogen concentration was calculated based on the

Minerals 2020, 10, 903 5 of 38

Table 1. An overview of the samples studied with some of their basic optical and infrared spectroscopic properties.

N/B Content, ppm Key Sample Cut, Weight Initial Color Type IR Characteristics Treatments Performed ( 5%) ± 2 MeV e- irradiation, TH 2.13 RBC, 0.086 ct F. yellowish brown IaB>>A 186 tot.; 31 A, 155 B AC4167, irr. peaks ≈ HT 800 ◦C TH 2.40 RBC, 0.080 ct F. olivish brown IaA>>B 186 tot.; 166 A, 20 B AC 4167/4063, H1b - ≈ TH 2.43 RBC, 0.097 ct F. dk. brown IaA>>B 199 tot.; 180 A, 19 B AC 4167 - ≈ TH 2.44 RBC, 0.067 ct F. yellowish brown IaB>>A 172 tot.; 39 A, 133 B AC 4167 - ≈ TH 2.47 RBC, 0.047 ct F. brown IaB>>A 300 tot.; 29 A, 271 B AC 4167 - ≈ AC 4167/4063, TH 2.48 RBC, 0.042 ct F.dk olive brown IaA>B 470 tot.; 317 A, 153 B - H1b, H1c TH 2.50 RBC, 0.125 ct F. brown yellow IaAB 12.6 tot.; 1.6 A, 12 B AC4167 - AC 3457/4112/4063, HT 950 C, 1050 C, 1500 C TH 2.52 RBC, 0.101 ct F.dp. yellow brown Ib/IaA 81 tot.; 45 A, 36 C ◦ ◦ ◦ H1b @2 h 2MeV e irr, HT TH 2.211 RBC, 0.116 ct Light brown IaB>>A 358 tot.; 18 A, 340 B AC 4190/4063 − 400–1310 ◦C 2MeV e irr, HT TH 2.212 RBC, 0.075 ct F. brown IaB>>A 98 tot.; 14 A, 84 B AC 4167/4063 − Deformation-related 400–1310 ◦C type Ia AC 4167/4063, TH 2.328 RBC, 0.062 ct F.dk. olive brown IaA>B 279 tot.; 205 A, 74 B HT 1500 C/2 h H1b, H1c ◦ AC 4167/4063, TH 2.330 RBC, 0.107 ct F.dk. brown IaA>B 330 tot.; 253 A, 77 B HT 950 C–1500 C @ 2 h H1b, H1c ◦ ◦ AC 4167/4063, TH 2.349 RBC, 0.034 ct F. dk. olive brown IaA>B 253 tot.; 140 A, 113 B HT 950 C–1050 C @ 2 h H1b, H1c ◦ ◦ 2MeV e irr. 2 h, HT TH 2.383 RBC, 0.089 ct F. brown IaA>B 262 tot.; 159 A, 103 B AC 4190/4063, H1b − 140–1250 ◦C 2MeV e irr. 2 h, HT TH 2.384 RBC, 0.095 ct F. light brown IaA>>B 99 tot.; 92 A, 7 B AC 4167 − 140–1250 ◦C 2MeV e irr. 2 h, HT TH 2.387 RBC, 0.121 ct F. brown IaB>A 59 tot.; 17 A, 42 B AC 4167 − 140–1250 ◦C 2MeV e irr. 2 h, HT TH 2.388 RBC, 0.099 ct F. brown IaAB 254 tot.; 114 A, 140 B AC 4190/4063, H1b − 140–1250 ◦C Minerals 2020, 10, 903 6 of 38

Table 1. Cont.

N/B Content, ppm Key Sample Cut, Weight Initial Color Type IR Characteristics Treatments Performed ( 5%) ± 10MeV e irr. 2 h, HT TH 2.389 RBC, 0.122 ct F. brown IaB>>A 200 tot.; 7 A, 193 B AC 4167 − 140–1250 ◦C 10MeV e irr. 2 h, HT TH 2.390 RBC, 0.076 ct F. dk brown IaB>>A 141 tot.; 19 A, 122 B AC 4167 − 140–1250 ◦C AC 4167/4063, 2MeV e irr. 2 h, HT TH 2.391 RBC, 0.129 ct F. dk. brown IaA>>B 307 tot.; 265 A, 42 B − H1b, H1c 140–1250 ◦C AC 4167/4063, 2MeV e irr. 2 h, HT TH 2.392 RBC, 0.110 ct F. brown IaA>>B 526 tot.; 477 A, 49 B − H1b, H1c 140–1250 ◦C TH 2.434 RBC, 0.154 ct Very light brown (N-O) IaB>A 45 tot.; 10 A, 35 B AC 4167 - TH 2.435 RBC, 0.137 ct Light brown (X-Y) IaA>B 175 tot.; 120 A, 55 B AC 4167 - TH 2.436 RBC, 0.146 ct Light brown (Z) IaA>B 50 tot.; 28 A, 22 B AC 4167/4063 - TH 2.437 RBC, 0.136 ct F.lt. brown IaA>B 59 tot.; 30 A, 29 B AC 4167/4063 - TH 2.438 RBC, 0.129 ct F. yellowish brown IaB>>A 93 tot.; 16 A, 77 B AC 4167 - TH 2.439 RBC, 0.154 ct F. brown IaA>B 220 tot.; 156 A, 64 B AC 4167 - Deformation-related TH 2.440 RBC, 0.143 ct F. pinkish brown IaA>>B 776 tot.; 660 A, 116 B AC 4167 - type Ia TH 2.441 RBC, 0.129 ct F. dk brown IaA>>B 134 tot.; 129 A, 5 B AC 4167 - TH 2.611 RBC, 0.113 ct F. brown IaA>>B 105 tot.; 95 A, 10 B AC 4167 - TH 2.613 RBC, 0.081 ct V.l. yellowish brown (O-P) IaA>>B 148 tot.; 125 A, 23 B AC 4167/4063 - TH 2.614 RBC, 0.176 ct V.l. yellowish brown (O-P) IaB>A 97 tot.; 30 A, 67 B AC 4190/4063 - TH 2.615 RBC, 0.078 ct Light brown (Q-R) IaB>A 89 tot.; 25 A, 64 B AC 4167/4063 - TH 2.617 RBC, 0.129 ct V.lt. brown (O-P) IaA>B 78 tot.; 52 A, 26 B AC 4190/4063 -

TH 2.619 RBC, 0.152 ct Light brown (X-Y) IaB 643 tot.; 643 B H rich, AC 4200 HT 1500 ◦C/2 h TH 2.620 RBC, 0.110 ct V.l. brown (O-P) IaB 40 tot.; 40 B AC 4200 - TH 2.621 RBC, 0.068 ct F. brown IaA>B 405 tot.; 265 A, 140 B AC 4167 - TH 2.622 RBC, 0.126 ct Lt. pinkish brown (S-T) IaA>>B 976 tot.; 892 A, 84 B H rich, AC 4167 - TH 2.623 RBC, 0.079 ct Lt. pinkish brown (S-T) IaA>>B 1049 tot.; 946 A, 103 B H rich, AC 4167 - EUR 001 RBC 0.027 ct F. yellowish brown IaB 320 tot.; 320 B H rich, AC 4200 - Minerals 2020, 10, 903 7 of 38

Table 1. Cont.

N/B Content, ppm Key Sample Cut, Weight Initial Color Type IR Characteristics Treatments Performed ( 5%) ± TH Ib-4 RBC, 0.101 ct F.dk. olive brown Ib>>IaA 14 tot.; 1 A, 13 C AC 4111 HPHT 1850 ◦C/5 min TH 2.51 RBC, 0.154 ct F. brown Ib 4.8 tot.; 4.8 C AC 4111 - Deformation-related TH 2.210 Prin, 0.153 ct F. yellowish brown Ib 6.2 tot.; 0.2 A, 6.0 C AC 4111 - type Ib TH 2.561 RBC, 0.135 ct F. brown Ib 2.0 tot.; 2.0 C AC 4167/4111 -

TH 2.46 RBC, 0.056 ct F. dk. olivish brown Ib 10 tot.; 3.5 A, 6.5 C AC 4111 HT 1500 ◦C/2 h

TH 2.332 RBC, 0.090 ct F. yellow brown IIa - AC 4063/4200 HT 1500 ◦C/2 h TH 2.445 RBC, 0.652 ct F. yellow brown IIa - AC 4063/4200 - TH 2.562 RBC, 0.062 ct Lt. yellowish brown (S-T) IIa - AC 4063/4200 - TH 2.563 RBC, 0.071 ct F.lt. yellowish brown IIa - AC 4063/4200 - Deformation-related TH 2.564 RBC, 0.068 ct F.lt. brown yellow IIa - AC 4063/4200 - type II TH 2.565 RBC, 0.037 ct Lt. yellowish brown (S-T) IIa - AC 4063/4200 -

TH 2.584 RBC, 0.017 ct F.lt. brown pink IIa - AC 4063/4200 HT 1500 ◦C/2 h TH 2.610 RBC, 0.083 ct F.lt. grayish yellowish brown IIb 0.05 (= 50 ppb) boron - - TH 2.612 RBC, 0.164 ct Lt. yellowish brown (O-P) IIa - AC 4063/4200 - TH 2.616 RBC, 0.090 ct Lt. yellowish brown (U-V) Near IIa <2 tot (A and B) AC 4063/4200 - 1.8 1017 neutrons/cm2, TH 2.163 RBC, 0.109 ct Colorless (E color) IaA>>B 144 tot.; 136 A, 8 B - × HT 1000 to 1500 ◦C, 2 h/step Type Ia colorless 1.8 1017 neutrons/cm2, (cape type) × HT 1000 to TH 2.290 Plate, 0.080 ct Colorless (F color) IaAB 225 tot.; 105 A, 120 B - 1500 ◦C, 2 h/step; HPHT 1850 ◦C/5 min.

e− = electron. Irr = irradiation. HT = annealed. HPHT = high pressure, high temperature treated. AC4167 = very weak AC; AC 4167 = weak AC; AC 4167 = medium AC; AC 4167 = strong AC. A = A center/A aggregate; B = B center/B aggregate; C = C center/single substitutional nitrogen. V.lt. = very light; Lt. = light; F.lt. = fancy light; F. = fancy; F.dp. = fancy deep; F.dk. = fancy dark. Minerals 2020, 10, x FOR PEER REVIEW 7 of 37

3. Results and Discussion

3.1. Natural Brown Diamonds Colored by Post-Growth Plastic Deformation-Related Defects

3.1.1. Type I Brown Diamonds Minerals 2020, 10, 903 8 of 38 A total of 40 type Ia and 5 type Ib diamonds of this category were analyzed in detail for this study. The samples represent all variations of such brown diamonds and they were carefully selected 3.to Resultsgive an accurate and Discussion view of the properties of the typical and the less common type I brown diamonds with a deformation-related color. 3.1. Natural Brown Diamonds Colored by Post-Growth Plastic Deformation-Related Defects

3.1.1.Microscopic Type I BrownExamination Diamonds AAll total diamonds of 40 type exhibited Ia and 5a typevariety Ib diamondsof inclusions, of this not category distinctly were different analyzed from in what detail one for expects this study. in near-colorless diamonds. These include fissures and solid inclusions like various crystals. In The samples represent all variations of such brown diamonds and they were carefully selected to give immersion the color distribution was found in all samples to be lamellar, consisting of brown and in an accurate view of the properties of the typical and the less common type I brown diamonds with a some cases pink narrow lamellae following the octahedral directions (Figure 2). These lamellae were deformation-related color. more or less obvious, but particularly in type Ib brown diamonds they were hardly discernible. When Microscopicobserved between Examination crossed polarizing filters, all diamonds showed very distinct gray to black extinction bands following the brown colored lamellae, plus distinct and bright interference colors, All diamonds exhibited a variety of inclusions, not distinctly different from what one expects in typical for diamonds that have experienced post-growth plastic deformation. The correlation near-colorless diamonds. These include fissures and solid inclusions like various crystals. In immersion between the color lamellae and lamellar extinction and their orientations as seen between the crossed the color distribution was found in all samples to be lamellar, consisting of brown and in some cases polarizing filters can be used to verify whether a diamond’s color is DR or NDR. An example can be pink narrow lamellae following the octahedral directions (Figure2). These lamellae were more or less seen in Figure 3, which is a hydrogen-rich type IaB brown diamond; the immersion images shown obvious, but particularly in type Ib brown diamonds they were hardly discernible. When observed prove that this stone is colored by DR defects and not by NDR defects. In many of the type Ia DR between crossed polarizing filters, all diamonds showed very distinct gray to black extinction bands brown diamonds, the strong light of the LED darkfield in use to observe the diamonds caused a following the brown colored lamellae, plus distinct and bright interference colors, typical for diamonds distinct green appearance of the diamonds because of green fluorescence (usually from the H3 that have experienced post-growth plastic deformation. The correlation between the color lamellae defect), an effect that—when strong enough—will result in what is known in the trade as a “green and lamellar extinction and their orientations as seen between the crossed polarizing filters can be used emitter” diamond. In none of the diamonds included in this study the “green emitter” effect was to verify whether a diamond’s color is DR or NDR. An example can be seen in Figure3, which is a pronounced enough to be visible face up under regular daylight. hydrogen-rich type IaB brown diamond; the immersion images shown prove that this stone is colored The hydrogen- and nitrogen-rich type IaA>>B samples included in this study were found to be by DR defects and not by NDR defects. In many of the type Ia DR brown diamonds, the strong of mixed cuboid-octahedral growth by fluorescence imaging as demonstrated; when verified under light of the LED darkfield in use to observe the diamonds caused a distinct green appearance of the crossed polarizing filters, the strain and extinction patterns on one hand follow the (111) brown diamonds because of green fluorescence (usually from the H3 defect), an effect that—when strong graining, and on the other hand are influenced by the cuboid and octahedral sectors, which cause enough—will result in what is known in the trade as a “green emitter” diamond. In none of the additional strain themselves (Figure 4). When comparing the immersion images and the fluorescence diamonds included in this study the “green emitter” effect was pronounced enough to be visible face images, it was apparent that the brown color lamellae traverse both octahedral and cuboid sectors up under regular daylight. uninterrupted, which is direct proof that the deformation is post-crystal growth.

Figure 2. AA pinkish pinkish brown brown type type IaA>>B IaA>> Bdiamond diamond (sample (sample TH TH 2.440) 2.440) in immersion in immersion (left) (left) showing showing the thecharacteristic characteristic plastic plastic deformation-related deformation-related brown brown “graining” “graining” along along (111) (111)combined combined with withsome somepink pink“graining”, “graining”, and andthe thecorresponding corresponding extinction extinction and and interference interference patterns patterns as as shown shown under crossed polarizingpolarizing ﬁlters.filters.

Minerals 2020, 10, 903 9 of 38 Minerals 2020, 10, x FOR PEER REVIEW 8 of 37

Minerals 2020, 10, x FOR PEER REVIEW 8 of 37

Figure 3.3. AA brownbrown hydrogen-rich hydrogen-rich type type IaB IaB diamond diamond (sample (sample TH TH 2.619) 2.619) that that clearly clearly shows shows its color its beingcolor causedbeing caused by deformation-related by deformation-related brown brown color lamellaecolor lamellae (left, immersion) (left, immersion) along thealong octahedral the octahedral crystal directions;crystal directions; the characteristic the characteristic extinction extinction patterns pa followingtterns following the brown the lamellaebrown lamellae (“graining”) (“graining”) can be seen can onbe seen the right on the (immersion, right (immers crossedion, polarizers).crossed polarizers).

The hydrogen- and nitrogen-rich type IaA>>B samples included in this study were found to be of mixed cuboid-octahedral growth by fluorescence imaging as demonstrated; when verified under crossed polarizing filters, the strain and extinction patterns on one hand follow the (111) brown graining,Figure and 3. A on brown the otherhydrogen-rich hand are type influenced IaB diamond by the (sam cuboidple TH and 2.619) octahedral that clearly sectors, shows whichits color cause additionalbeing caused strain themselvesby deformation-related (Figure4). Whenbrown comparingcolor lamellae the (left, immersion immersion) images along and the the octahedral fluorescence images,crystal it wasdirections; apparent the characteristic that the brown extinction color pa lamellaetterns following traverse the both brown octahedral lamellae (“graining”) and cuboid can sectors uninterrupted,be seen on the which right is (immers direction, proof crossed that thepolarizers). deformation is post-crystal growth.

Figure 4. A brown hydrogen-rich type IaA>>B diamond (sample TH 2.622) that clearly shows its color being caused by deformation-related brown color lamellae (left, immersion) along the octahedral crystal directions, the color lamellae traversing the cuboid growth sectors that were visualized by fluorescence imaging (see Figure 21 below); the characteristic extinction patterns following the brown lamellae (“graining”) can be seen on the right, but additionally to the (111) extinction, some additional extinction/strain patterns can be seen from the strain caused between the different growth sectors (immersion, crossed polarizers).

Infrared and Near-Infrared Spectroscopy FigureFigure 4. 4. AA brown brown hydrogen-rich hydrogen-rich type type IaA>>B IaA>> diamondB diamond (sample (sample TH 2.622) TH 2.622) that clearly that clearly shows shows its color its beingcolorThe infrared beingcaused caused byspectra deformation-related by deformation-related of the plastically brown deformed brown color color lamellae type lamellae Ia (left,brown (left, immersion) immersion) diamonds along along included the the octahedral octahedral in this study characterizedcrystalcrystal directions, directions, them as the the diamonds color color lamellae lamellae with traversing traversinga usually the therather cuboid cuboid low growth growth nitrogen sectors sectors and that thathydrogen were were visualized visualized content, by bywith a low aggregationfluorescencefluorescence imaging state (hence (see Figure mostly 21 below); below);type IaA), the the charac characteristicparticularlyteristic extinction extinctionfor the darker-colored patte patternsrns following following diamonds, the the brown brown while the lighter-coloredlamellaelamellae (“graining”) (“graining”) specimens can be be seenwere seen on on very the the right, right,variable but but adin additionally ditionallynitrogen to toaggregation, the the (111) (111) extinction, extinction, from IaA some some to additional additionalIaB (Figures 5 and extinction/strain6).extinction The nitrogen/strain patterns patterns content can can ranged be be seen seen from from from ≈the the78 st straintorain ≈526 caused caused ppm between between (±5%), the theand different di onlyfferent samples growth sectors containing significant(immersion,(immersion, concentrations crossed polarizers). of hydrogen were found to contain more nitrogen, with the highest concentration having been determined at ≈1049 ppm in an unusual diamond being both H-rich as InfraredInfrared and and Near-Infrared Near-Infrared Spectroscopy well as plastically deformed. Hydrogen-rich DR brown diamonds are rare, and in most DR brown diamondsTheThe infrared infrared the hydrogen spectra spectra is of of detectable, the the plastically plastically but only deformed deformed weakly. type type The Ia Ia nitrogen brown brown diamonds diamondsconcentration included included was found in in this this to study study have characterizedcharacterizedno influence whatsoever them asas diamondsdiamonds on how with darkwith a ora usually usuallylight a rather brown rather low diamondlow nitrogen nitrogen was. and and hydrogen hydrogen content, content, with with a low a lowaggregation aggregationThe purely state brownstate (hence (hence type mostly Ibmostly diamonds type type IaA), IaA), analyzed particularly particularly all had for theforvery darker-coloredthe low darker-colored nitrogen diamonds,contents diamonds, between while while the≈2 thelighter-coloredand lighter-colored ≈10 ppm, with specimens specimensthe one were sample were very that very variable represents variable in nitrogen in th nitrogene more aggregation, saturated aggregation, from deep IaAfrom yellow to IaA IaB brown to (Figures IaB group(Figures5 and (TH6 ).5 The nitrogen content ranged from 78 to 526 ppm ( 5%), and only samples containing significant and2.52) 6). being The distinctly nitrogen highercontent in ranged nitrogen≈ from with≈ ≈ 78≈82 to ppm ≈526± total ppm nitrogen (±5%), content.and only With samples exception containing of that significantconcentrationshigher nitrogen concentrations ofIb hydrogendiamond, of none werehydrogen of found them were toshowed contain found any more toY centercontain nitrogen, one-phonon more with nitrogen, the absorption. highest with concentration Thethe Yhighest center having been determined at 1049 ppm in an unusual diamond being both H-rich as well as plastically concentrationwith its main absorptionhaving been at ≈ determined1145 cm−1 is atan ≈ extremely1049 ppm commonin an unusual absorption diamond that beingoccurs both together H-rich with as welldeformed.the X as center plastically Hydrogen-rich (main deformed. absorption DR brownHydrogen-rich at 1332 diamonds cm−1) inDR arenatural brown rare, andtype diamonds in Ib most diamonds—particularly DRare brown rare, and diamonds in most thein DR hydrogenyellow brown to diamondsorange stones the hydrogen[18]. In none is detectable, of them, with but onlythe exce weakptionly. The of sample nitrogen TH concentration 2.52, hydrogen was was found detectable; to have nothe influence explanation whatsoever for that is on found how darkin the or fact light that a brownthe Y center diamond is directly was. associated with hydrogen in The purely brown type Ib diamonds analyzed all had very low nitrogen contents between ≈2 and ≈10 ppm, with the one sample that represents the more saturated deep yellow brown group (TH 2.52) being distinctly higher in nitrogen with ≈82 ppm total nitrogen content. With exception of that higher nitrogen Ib diamond, none of them showed any Y center one-phonon absorption. The Y center with its main absorption at 1145 cm−1 is an extremely common absorption that occurs together with the X center (main absorption at 1332 cm−1) in natural type Ib diamonds—particularly in yellow to orange stones [18]. In none of them, with the exception of sample TH 2.52, hydrogen was detectable; the explanation for that is found in the fact that the Y center is directly associated with hydrogen in

Minerals 2020, 10, 903 10 of 38 is detectable, but only weakly. The nitrogen concentration was found to have no inﬂuence whatsoever on how dark or light a brown diamond was. The purely brown type Ib diamonds analyzed all had very low nitrogen contents between 2 and ≈ 10 ppm, with the one sample that represents the more saturated deep yellow brown group (TH 2.52) ≈ being distinctly higher in nitrogen with 82 ppm total nitrogen content. With exception of that higher ≈ nitrogen Ib diamond, none of them showed any Y center one-phonon absorption. The Y center with 1 its main absorption at 1145 cm− is an extremely common absorption that occurs together with the X 1 center (main absorption at 1332 cm− ) in natural type Ib diamonds—particularly in yellow to orange stonesMinerals [202018]., 10 In, x noneFOR PEER of them, REVIEW with the exception of sample TH 2.52, hydrogen was detectable;9 of the 37 explanation for that is found in the fact that the Y center is directly associated with hydrogen in type Ib diamonds,type Ib diamonds, while the while Ccenter the C center is not is [17 not,19 [17,19];]; hence, hence, pure pure type type Ib diamonds Ib diamonds without without the Ythe center Y center do neverdo never contain contain IR activeIR active hydrogen. hydrogen. The spectra ofof allall samples,samples, type type Ia Ia and and type type Ib, Ib show, show amber amber center center (abbreviated (abbreviated with with “AC” “AC” in the in text)the text) absorptions, absorptions, with with either either one, on two,e, two, or more or more ZPLs ZPLs (Figure (Figure5). 5).

The Room Temperature IRIR SpectroscopySpectroscopy ofof thethe AmberAmber CentersCenters In type Ia diamonds with at least some A aggregatesaggregates detectable, the following AC ZPLs were 1 1 found in thethe roomroom temperaturetemperature IRIR spectra:spectra: 40654065 andand 4165 4165 cm cm−−1 with small 35203520 andand 27602760 cmcm−−1 peaks 1 detectable in samples with intense 4165 cm−1 ACAC absorption absorption (Figure (Figure 55,, tracestraces a,a, b,b, andand c).c).

Figure 5. Representative normalized infrared infrared spectra showing the different diﬀerent types of amber centers (ACs) in brown diamonds, recorded at room temperature.temperature. Type Ia diamonds show either an AC with −1 −11 the main absorptionabsorption atat 41654165 cmcm− oror a “double AC” having a double bandband atat 41654165 andand 40654065cm cm− (traces −1 1 −1 1 a, b, and c), with weak bands bands at at 3520 3520 and and 2760 2760 cm cm− whenwhen the the 4165 4165 cm cm −ACAC is strong; is strong; in inthe the spectra spectra of −1 1 oftype type Ib diamonds, Ib diamonds, there there is a is eith a eitherer a single a single main main band band at 4112 at 4112 cm cm or− a doubleor a double band band at 4165 at 4165and 4112 and −1 1 −1 1 −1 1 4112cm , cmsometimes− , sometimes with a with 3462 a cm 3462 and cm −a 2915and acm 2915 AC cm (traces− AC e, (traces f, and e, g); f, andin some g); in type some Ib typeDR brown Ib DR −1 1 browndiamonds, diamonds, a band at a band4240 cm at 4240 is distinct; cm− is it distinct; is unclear it is whether unclear it whether is another it isAC another zero phonon AC zero line phonon (ZPL) −1 1 lineor a (ZPL)component or a component of the 3462 ofcm the AC 3462 (traces cm− e ACand (traces g). The e spectra and g). we There spectrashifted vertically were shifted for verticallyclarity. for clarity. As expected from earlier work [6,8], the amber center absorption is most intense in dark samples of type IaA, while it is weak when the color is light and/or the content of the A aggregates is low compared to the content in B aggregates (Figure 6). In the pure type IaB brown diamonds included in this study, the ACs described above were absent and instead a weak band at 4200 cm−1 was found (Figure 5, trace d). Some spectra were observed with the 4065 cm−1 AC and the second AC at 4190 cm−1 instead of 4165 cm−1; these were samples with significantly more B aggregates than A aggregates, and the shifted position is interpreted as a combination of two bands, the 4165 cm−1 AC and the 4200 cm−1 AC. From the underlying asymmetry of the 4165 cm−1 absorption in the spectra of many diamonds studied, it can be concluded that the 4200 cm−1 AC is underlying many of the 4165 cm−1 AC ZPLs.

Minerals 2020, 10, 903 11 of 38

As expected from earlier work [6,8], the amber center absorption is most intense in dark samples of type IaA, while it is weak when the color is light and/or the content of the A aggregates is low compared to the content in B aggregates (Figure6). In the pure type IaB brown diamonds included in 1 this study, the ACs described above were absent and instead a weak band at 4200 cm− was found 1 1 (Figure5, trace d). Some spectra were observed with the 4065 cm − AC and the second AC at 4190 cm− 1 instead of 4165 cm− ; these were samples with signiﬁcantly more B aggregates than A aggregates, 1 and the shifted position is interpreted as a combination of two bands, the 4165 cm− AC and the 1 1 4200 cm− AC. From the underlying asymmetry of the 4165 cm− absorption in the spectra of many 1 1 diamonds studied, it can be concluded that the 4200 cm− AC is underlying many of the 4165 cm− ACMinerals ZPLs. 2020, 10, x FOR PEER REVIEW 10 of 37

Figure 6.6. The normalized infrared spectra of a color suite of diamondsdiamonds with aa deformation-relateddeformation-related brown color. It can be seen that in these type Ia diamonds, diamonds, the intensity of the AC absorption absorption correlates well with the darkness of the brown color;color; thethe lightestlightest brown diamondsdiamonds have the weakest AC while the darkestdarkest brownbrown diamondsdiamonds havehave thethe strongeststrongest ACAC absorption.absorption. The spectra were shifted vertically for clarity.

All type Ib DR brownbrown diamondsdiamonds except sample TH 2.52 showedshowed a veryvery distinctdistinct ACAC absorptionabsorption 1 with a broadened and asymmetr asymmetricalical ZPL visible visible at at 4112 4112 cm cm−−1 atat RT RT (room (room temperature), temperature), sometimes sometimes a 1 1 a4165 4165 cm cm−1 AC− AC (Figure (Figure 5, traces5, traces f and f and g), plus g), plus a ZPL a ZPL at 3462 at 3462 cm−1 cm. In −the. spectra In the spectra of many of of many the type of the Ib 1 typediamonds, Ib diamonds, a somewhat a somewhat broader broaderabsorption absorption at 4240 cm at 4240−1 was cm visible,− was which visible, always which occurred always occurredtogether 1 togetherwith the 3462 with cm the−1 3462 AC; cmat room− AC; temperature, at room temperature, the AC of sample the AC TH of sample2.52 exhibited TH 2.52 almost exhibited exclusively almost 1 exclusivelythese two absorptions, these two absorptions, with a weak with band a weak at 4065 band cm at−1 4065 (Figure cm− 5,(Figure trace e).5, traceAdditional, e). Additional, often hidden often 1 hiddenbands at bands 2790, at 2880, 2790, and 2880, 2915 and 2915cm−1 cmwere− weredetected detected in some in some of the of the spectra spectra inin type type Ib Ib DR DR brown 1 diamonds; they always occurredoccurred inin combinationcombination withwith thethe ACAC atat 34623462 cmcm−−1. This This band could only be properly visualizedvisualized byby subtractionsubtraction ofof thethe intrinsicintrinsic diamonddiamond absorptionsabsorptions (Figure(Figure7 7).). A link between these absorptions could be excluded since in the spectrum of sample TH 2.52 a 1 1 distinct band atat 34623462 cmcm−−1 bandband could could be be seen seen together with the possibly associated bandband atat 42404240 cmcm−−1,, 1 but the bands at 28802880 andand 29152915 cmcm−−1 werewere absent. absent. Furthermore, Furthermore, the the same same was true for more than half of thethe 27 27 type type Ib diamondsIb diamonds (of various (of various brownish browni colors),sh showingcolors), obviousshowing signs obvious of plastic signs deformations of plastic thatdeformations were tested that during were tested the PhD duri thesisng the of PhD TH [thesis19], which of TH all [19], exhibited which all IR exhibited spectra with IR spectra the band with at 1 1 1 approximatelythe band at approximately 3462 cm− . In 3462 every cm one−1. In of every them theone band of them at 3462 the cmband− was at 3462 associated cm−1 was by theassociated 4240 cm by− 1 band,the 4240 but cm in− contrast,1 band, but the in bands contrast, at 2880 the and bands 2915 at cm 2880− were and 2915 present cm in−1 were less than present half in of thoseless than diamonds. half of those diamonds. Looking at the detail of the spectra it was found that the 2880 and 2915 cm−1 bands appeared to correlate with the bands at 4510 and 4540 cm−1. The data indicates that these bands at 2880 and 2915 cm−1 represent at least two more AC ZPLs. It must be noted that the 3462 cm−1 absorption should not be confused with the 3475 cm−1 absorption present in the spectra of high nitrogen and hydrogen type IaA>>Ib diamonds of mixed cuboid-octahedral growth [20]. The two absorptions may look similar but are totally unrelated, and besides the slightly different position, the 3475 cm−1 absorption also has a distinctly lower FWHM.

Minerals 2020, 10, 903 12 of 38

1 Looking at the detail of the spectra it was found that the 2880 and 2915 cm− bands appeared to 1 correlate with the bands at 4510 and 4540 cm− . The data indicates that these bands at 2880 and 1 2915 cm− represent at least two more AC ZPLs. 1 1 It must be noted that the 3462 cm− absorption should not be confused with the 3475 cm− absorption present in the spectra of high nitrogen and hydrogen type IaA>>Ib diamonds of mixed cuboid-octahedral growth [20]. The two absorptions may look similar but are totally unrelated, and besides the slightly diﬀerent position, the 3475 cm 1 absorption also has a distinctly lower FWHM. Minerals 2020, 10, x FOR PEER REVIEW − 11 of 37

Figure 7. TheThe normalized normalized near near infrared infrared sp spectraectra of type type Ib Ib DR DR brown brown diamonds diamonds after subtraction of the −11 intrinsic diamond absorptions. Al Alll of the spectra show thethe 34623462 cmcm− ACAC ZPL ZPL and and its likely vibronic −11 −1 component at 4240 cmcm− ;; this this graph graph demonstrates demonstrates that the bands betweenbetween 29152915 andand 27902790 cmcm− do not 1−1 relate toto thethe 34623462 cm cm− absorption.absorption.Instead, Instead, it it is is indicated indicated that that these these bands bands are are additional additional ACs, ACs, and and that 1 −1 thatthe bands the bands at 4540 at and4540 4510 and cm4510− arecm likely are likely vibronic vibronic components components of these of ACs.these The ACs. spectra The spectra were shifted were shiftedvertically vertically for clarity. for clarity.

Low Temperature NIRNIR SpectraSpectra ofof thethe AmberAmber CentersCenters 1 The NIR spectra recorded at 77 K of the various ACsACs fromfrom 40004000 cmcm−−1 cancan be be seen seen in in Figure 88 andand a complete view view showing showing the the entire entire vibronic vibronic structure structure of of the the different diﬀerent ACs ACs can can be beseen seen in Figure in Figure 9. In9. 1 1 spectraIn spectra with with a simple a simple 4165 4165 cm cm−1 AC,− AC, the theZPL ZPL was was found found to only to only sharpen sharpen andand shift shift to 4167 to 4167 cm−1 cm at− a 1 1 lowat a lowtemperature. temperature. In the In spectra the spectra where where both botha 4165 a4165 cm−1 cmAC− andAC a and 4065 a 4065cm−1 cmAC− wereAC werepresent, present, both 1 1 1 ZPLsboth ZPLsgot sharper got sharper and shifted and shifted to 4167 to cm 4167−1 and cm 4063− and cm 4063−1, while cm− a ,new while ZPL a newat 4092 ZPL cm at−1 4092was detectable cm− was 1 (Figuredetectable 8, traces (Figure a8 and, traces b). aFor and the b). type For Ib the diamonds type Ib diamonds with an withisolated an isolatedAC at 4112 AC atcm 4112−1 at cmRT,− thisat 1 1 absorptionRT, this absorption sharpened, sharpened, shifted shiftedslightly slightly to 4111 to cm 4111−1, and cm− several, and several ZPLs appeared ZPLs appeared at 4063 at 4063cm−1, cm4136− , 1 1 1 cm4136−1, cmand− ,4167 and cm 4167−1 cm(Figure− (Figure 8, traces8, traces c, d, c, and d, and e). The e). The 4240 4240 cm cm−1 absorption− absorption sharpened sharpened somewhat somewhat in 1 1 thein the diamond diamond with with a dominant a dominant 4240 4240 cm cm−1−/3462/3462 cm cm−1 −featurefeature (Th (Th 2.52), 2.52), but but otherwise otherwise remained remained pretty pretty much unchanged in all other samples, and hence maintained the same intensity and FWHM. This 1 indicates that the peak is unlikely anotheranother ACAC ZPL,ZPL, butbut ratherrather aa featurefeature associatedassociated withwith thethe 34623462 cmcm−−1 AC. TheThe NIRNIR spectrum spectrum recorded recorded at at 77 77 K ofK Sampleof Sample TH 2.52TH was2.52 unusualwas unusual as the as low the temperature low temperature caused 1 1 1 causedseveral ACsseveral to appearACs to in appear signiﬁcant in significant intensity, namelyintensity, the namely ACs at the 4063 ACs cm −at, 4063 4092 cmcm−−1,, 4092 and 4111cm−1 cm, and− . 1 4111 Ourcm−1. low temperature setup did unfortunately not permit to include the range to 2600 cm− , 1 1 1 1 henceOur we low were temperature unable to resolvesetup did the unfortunately true shape of not the permit 3520 cm to− include, 3462 cmthe− range, 2915 to cm 2600− , cm 2880−1, hence cm− , 1 weand were 2760 unable cm− bands to resolve and see the how true they shape behaved of the at 3520 77 K. cm For−1, the3462 rest cm of−1, this 2915 study, cm−1, the 2880 AC cm ZPL−1, and positions 2760 cm−1 bands and see how they behaved at 77 K. For the rest of this study, the AC ZPL positions will be indicated at LNT (liquid nitrogen temperature), with the exception of the bands measured only at RT (3520 cm−1, 3462 cm−1, 2915 cm−1, 2880 cm−1, and 2760 cm−1).

Minerals 2020, 10, 903 13 of 38 will be indicated at LNT (liquid nitrogen temperature), with the exception of the bands measured only 1 1 1 1 1 atMinerals RT (3520 2020, cm10, −x FOR, 3462 PEER cm REVIEW− , 2915 cm− , 2880 cm− , and 2760 cm− ). 12 of 37

FigureFigure 8.8. AA detaileddetailed viewview ofof thethe ACAC ZPLsZPLs whenwhen recordedrecorded atat 7777 K.K. AA totaltotal ofof 55 ZPLsZPLs cancan bebe seenseen andand the band at 4240 cm−1 is very likely identified as a vibronic component, since it does not sharpen the band at 4240 cm− is very likely identiﬁed as a vibronic component, since it does not sharpen signiﬁcantly.significantly. A A distinct H1b center can be seen inin spectrumspectrum (a);(a); whenwhen presentpresent inin diamondsdiamonds notnot associatedassociated with any any type type of of irradiation/annealing, irradiation/annealing, th thisis peak peak is always is always associated associated with with the AC the at AC 4063 at cm−1. The1 spectra were shifted vertically for clarity. 4063 cm− . The spectra were shifted vertically for clarity.

−1 −11 TheThe 41364136 cmcm− absorption appears unique toto typetype IbIb diamonds.diamonds. Curiously,Curiously,the the 4063 4063 cm cm− asas wellwell −1 −11 asas thethe 41674167 cmcm− absorptions were detected in samplesample THTH 2.51,2.51, andand aa ratherrather strongstrong4167 4167cm cm− ACAC −11 absorptionabsorption waswas detecteddetected inin combinationcombination withwith aa 41114111 cmcm− AC absorption in sample THTH 2.561,2.561, whichwhich bothboth are—accordingare—according toto theirtheir one-phononone-phonon IRIR absorption—veryabsorption—very lowlow nitrogennitrogen purepure typetype IbIb diamondsdiamonds withwith aggregatedaggregated nitrogennitrogen undetectable.undetectable. WhileWhile oneone couldcould interpretinterpret thisthis asas nitrogennitrogen possiblypossibly notnot atat allall involvedinvolved inin thesethese defects, defects, it it is is more more likely likely that that extremely extremely low low nitrogen nitrogen concentrations concentrations are are su ffisufficientcient to createto create detectable detectable signals signals of the of ACs. the While ACs. aggregatedWhile aggregated nitrogen wasnitrogen undetectable was undetectable in the IR spectrum in the ofIR thespectrum given samples,of the given the N3 samples, and H3 the centers N3 wereand H3 detectable centers bywere PL spectroscopy.detectable by ThisPL spectroscopy. means that at leastThis somemeans A that and Bat aggregates least some must A and have B beenaggregates present, must although have most been of present, the A aggregates although could most have of the been A usedaggregates up in thecould H3 have defects. been The used concentration up in the H3 of defe nitrogencts. The aggregates concentration was well of nitrogen below 1 ppm,aggregates otherwise was 1 1 wewell would below have 1 ppm, been otherwise able to measure we would it by have FTIR been spectroscopy. able to measure With theit by 4167 FTIR cm spectroscopy.− and 4111 cm With− ACs the being4167 cm absent−1 and or 4111 weak cm in−1 ACs DR typebeing IIa absent and IIb or diamonds,weak in DR nitrogen type IIa and clearly IIb seemsdiamonds, to play nitrogen a role clearly in the defectseems structure,to play a butrole from in the the defect above structure, observation but it from appears the reasonableabove observation to suggest it thatappears very reasonable low nitrogen to concentrationssuggest that very are low suffi nitrogencient to createconcentrations significant are AC su absorption.fficient to create It also significant seems reasonable AC absorption. to assign It 1 1 1 −1 1 −1 1 −1 −1 −1 thealso 2760 seems cm reasonable− , 3520 cm to− assign, 4063 cmthe− 2760, 4092 cm cm, −3520, and cm 4167, 4063 cm −cmZPLs, 4092 to cm vacancy, and defects 4167 cm containing ZPLs to 1 Avacancy aggregates. defects The containing ACs only A detectedaggregates. in diamondsThe ACs on containingly detected C in centers diamonds with containing ZPLs at 2790 C centers cm− , 1 1 −1 1−1 −11 −1 1 −1 −1 2880with cmZPLs− , at 2915 2790 cm cm− , 3462, 2880 cm cm− ,, 4111 2915 cm cm− , and3462 4136 cm cm, 4111− are cm either, and related4136 cm to Care center-containing either related to vacancyC center-containing defects, or the vacancy more likely defects, possibility or the more is that lik theyely representpossibility di isfferent that chargethey represent states of different the ACs 1 1 1 −1 1 −1 −1 1 −1 −1 withcharge ZPLs states at 2760 of the cm ACs− , 3520 with cm ZPLs− , 4063at 2760 cm −cm, 4092, 3520 cm cm− , and, 4063 4167 cm cm, −4092. Since cm the, and C center4167 cm acts. asSince an electronthe C center donor, acts it canas an cause electron the change donor, of it the can charge cause statesthe change of the of defects the charge as, for states example, of the they defects form H2as, 0 − 0 (N-V-N)for example,− in H3 they (N-V-N) form H2-containing (N-V-N) in diamonds H3 (N-V-N) through-containing this mechanism. diamonds through this mechanism. Only the 4200 cm−1 band appeared to be the ZPL of an AC related to the B aggregates; in contrast to the other ACs absorptions measured at LNT, the 4200 cm−1 AC sharpened less distinctly at 77 K. Only some bands clearly relate to a specific AC ZPL, others can only be tentatively assigned to a specific ZPL since there appears to be significant overlap between many of the structures associated to the different ACs (Table 2).

Minerals 2020, 10, 903 14 of 38

1 Only the 4200 cm− band appeared to be the ZPL of an AC related to the B aggregates; in contrast 1 to the other ACs absorptions measured at LNT, the 4200 cm− AC sharpened less distinctly at 77 K. Only some bands clearly relate to a specific AC ZPL, others can only be tentatively assigned to a specific ZPL since there appears to be significant overlap between many of the structures associated to the different ACs (Table2). Minerals 2020, 10, x FOR PEER REVIEW 13 of 37

Figure 9.9. TheThe NIRNIR spectra spectra of of the the di ffdifferenterent AC AC absorptions absorption recordeds recorded with thewith samples the samples at 77 K. at The 77 spectraK. The confirmspectra confirm which ones which of the ones absorptions of the absorptions are ZPL’s are and ZPL’s which and ones which are not, ones and are the not, significant and the sharpeningsignificant andsharpening splitting and of splitting the absorptions of the absorptions reveal much reveal more much details more about details the about ACs thanthe ACs the spectrathan the at spectra room temperature.at room temperature. The spectra The werespectra shifted were vertically shifted vertically for clarity. for clarity.

Table 2.2. The different diﬀerent AC ZPLs and tentative details of th theireir associated structure and spectral features.

ZPLZPL Likely Likely Associated Associated Structure Structure and and otherother AssociatedAssociated Spectral Spectral Features Features (LNT) (LNT) 48544854 (ZPL?), (ZPL?), 5990,5990, 5890, 5890, 7919 7919 42004200 (LNT) (LNT) 6700 6700 81158115 8410 8410 88108810 51155115 57705770 (ZPL?) (ZPL?) 41674167 (LNT) (LNT) 4673 4673 5855 5855 7305 7305 7760 7760 8020 8020 8530 8530 41364136 (LNT) (LNT) Too Too weak weak to to determine determine associatedassociated structure structure 41114111 (LNT) (LNT) 5875 5875 7265 7265 8020 8020 8530 8530 9325 9325 40924092 (LNT) (LNT) Too Too weak weak to to determine determine associatedassociated structure structure 4063 (LNT) 5875 7305 8080 8530 4063 (LNT) 5875 7305 8080 8530 3520 (RT) Too weak to determine associated structure 3520 (RT) Too weak to determine associated structure 3462 (RT) 4240 5850 7960 29153462 (RT) (RT) 4240 5850 7960 4525 28802915 (RT) (RT) 4525? 4525 27902880 (RT) (RT) Too weak to determine 4525? associated structure 27602790 (RT) (RT) Too Too weak weak to to determine determine associatedassociated structure structure 2760 (RT) Too weak to determine associated structure The Photoexcitation Behavior of the Amber Centers The photoexcitation experiments performed during this study and during the PhD thesis of TH have shown that excitation with 360 nm light causes certain ACs to increase while others decrease. This at first sight random phenomenon was found to be most prominent in type Ib diamonds, but to some degree detectable in all diamonds with ACs. A detailed look at as many samples as possible, including the ones of this study and all samples (including non-brown diamonds with ACs) included in the PhD thesis of TH and the ones used for an irradiation study currently performed by the authors, has shed more light on the photoexcitation behavior of the ACs.

Minerals 2020, 10, 903 15 of 38

The Photoexcitation Behavior of the Amber Centers The photoexcitation experiments performed during this study and during the PhD thesis of TH have shown that excitation with 360 nm light causes certain ACs to increase while others decrease. This at ﬁrst sight random phenomenon was found to be most prominent in type Ib diamonds, but to some degree detectable in all diamonds with ACs. A detailed look at as many samples as possible, including the ones of this study and all samples (including non-brown diamonds with ACs) included in the PhD thesis of TH and the ones used for an irradiation study currently performed by the authors, has shed more light on the photoexcitation behavior of the ACs. 1 The conclusions drawn from this work are that the 4167/4111/4063 cm− ACs decrease in the IR 1 1 1 spectra that do not show the 3462 cm− /2790 cm− AC features. The 4167/4111/4063 cm− ACs increase 1 whenever the beforementioned features are present in the spectra; while the 4167/4111/4063 cm− ACs 1 1 increase, the 3462 cm− /2790 cm− AC peaks decrease or even disappear. Illuminating the samples by wavelengths >400 nm for a short time restores the original appearance of the ACs. Another result from the photoexcitation experiments was obtained from sample TH 2.52, the type 1 1 1 Ib/IaA yellow brown diamond with—besides a faint 4063 cm− AC—only the 4240 cm− and 3462 cm− 1 bands detectable at RT: photoexcitation caused the 3462 cm− to reduce signiﬁcantly, while two distinct 1 1 ACs at 4100 (a mix of the 4111 cm− AC and the 4092 cm− AC as determined at 77 K above) and 1 1 4063 cm− appeared; the 4240 cm− feature did not change during this experiment. This again is 1 indicative that the 4240 cm− absorption is not a ZPL, but a vibronic component of another center, 1 likely the 3462 cm− AC.

Naturally Occurring H1b and H1c Centers in Brown Diamonds with Amber Centers As seen in Figure8 (trace a) and Figure5 (trace a, b, and e), type I DR brown diamonds can exhibit natural H1b and the H1c centers; these defects are known in any diamond that contains aggregated nitrogen after irradiation and annealing from 550 ◦C to about 1400 ◦C[21]. In the brown diamonds they are only detectable in type I diamonds that contain aggregated nitrogen and that exhibit IR spectra 1 that show the 4063 cm− AC; in most of them only a very weak H1b peak can be found, H1c is rare. Years back we believed that the peak detected in such stones was just part of the vibronic structure of 1 the 4063 cm− AC, but a range of tests have led to the conclusion that the defect responsible is indeed H1b; this has been further confirmed when samples were found with detectable H1c absorption. A detailed analysis of a series of DR brown diamonds with H1b and of some with both H1b and H1c has shown that there is no indication whatsoever for natural or artificial irradiation and annealing in such diamonds; their spectroscopic data does not vary in any specific way from other type Ia DR brown diamonds. Clearly, in such brown diamonds the H1b and H1c defects are formed while plastic deformation creates high densities in dislocations and vacancies. One could speculate that aggregated nitrogen—A aggregates in H1b, B aggregates in H1c [21], combined with an extended vacancy defect, consisting of at least several vacancies—results in these defects. In irradiated diamonds, monovacancies but also extended vacancies are induced by irradiation; monovacancies combine with A and B aggregates to form H3 and H4, but their formation does not at all correlate with the formation of H1b and H1c; the 595 nm defect has been suggested to correlate with the formation of H1b and H1c, and currently the cause of this defect is unknown [21]. The brown diamonds with H1b and H1c do not exhibit the 595 nm defect, hence one might wonder if H1b and H1c actually relate in any way to the 595 nm defect. A more complex vacancy defect must be involved in the H1b and H1c defects; however, it cannot be the large vacancy cluster, because of the high thermal stability and inertia of the 1 vacancy cluster defect. The annealing behavior of the different ACs, including the 4063 cm− AC and the related H1b and H1c defects, was therefore verified during this study, see results below. Minerals 2020, 10, 903 16 of 38

The Annealing Behavior of the Amber Centers and Naturally Occurring H1b/H1c While all ACs and natural H1b in brown diamonds have been shown to anneal out by HPHT treatment from about 2000 ◦C[19], limited and incomplete data on the annealing behavior of the ACs at a lower temperature is available [22], while no data on the behavior of natural H1b at a lower temperature have been published, and no data whatsoever on the annealing behavior of natural H1c have been published so far. Therefore, some samples with IR spectra representing the different ACs and samples with H1b and H1c were annealed up to 1500 ◦C in this study, and some were treated by HPHT at 1850 ◦C up to 2250 ◦C. These high temperature annealing experiments have shown that heating diamonds with ACs at 1 1 1 4167/4063/4092 (LNT) to 1500 ◦C annealed out the 4063 cm− and 4092 cm− AC while the 4167 cm− AC increased distinctly (Figure 10). In the samples used for this specific experiment (TH 2.328 and 1 TH 2.330), in the spectra measured at room temperature, the 4167 cm− AC ZPL increased by a factor of 1 1 3.5 after two hours of heating at 1500 ◦C while the 4063 cm− peak disappeared; at 77 K, the 4063 cm− 1 AC was barely detectable or undetectable and the 4092 cm− AC was undetectable. These diamonds also exhibited weak but clearly visible H1b and H1c absorptions prior to the annealing, and both annealed out at 1500 ◦C (Figure 10). To determine the precise annealing behavior of the ACs in 1 1 diamonds showing both the 4167 cm− and 4063 cm− ACs, sample TH 2.330 was heated from 950 to 1350 ◦C in 100 ◦C steps and from 1350 ◦C to 1500 ◦C in 50 ◦C steps, for two hours at every step. This treatment has shown that neither the AC nor H1b and H1c were affected up to a temperature 1 of 1050 ◦C; from 1150 ◦C onwards the 4063 cm− AC got weaker, until it disappeared at 1500 ◦C, 1 while the 4167 cm− AC increased, until it was strongest at 1500 ◦C. H1b and H1c started to increase from 1150 ◦C to 1250 ◦C, when their intensity was found to be 4 to 5 times that of their intensity in the unheated state; then both decreased until they were both undetectable at exactly 1500 ◦C. Hence, it was 1 shown that the temperature when the 4063 cm− AC and H1b H1c completely anneal out is identical. 1 The exact annealing behavior appears to correlate to when the 4063 cm− AC starts to decrease in intensity, and H1b and H1c increase significantly; the H1b and H1c defects in these brown diamonds 1 1 could be interpreted as a side product of the transformation of the 4063 cm− AC into the 4167 cm− 1 and/or 4111 cm− AC. This interpretation is supported by the fact that the H1b and H1c absorptions in the brown diamonds do not increase in intensity before 1150 ◦C. In the artificially irradiated diamonds of type I studied for various research projects by our laboratory, the common behavior of H1b and H1c is that they are detected between 600 and 750 ◦C and that these increase to different temperatures, somewhere between 1100 and 1200 ◦C, when they typically start to decrease and anneal out between 1400 ◦C and 1500 ◦C. The situation looks somewhat different for very heavily irradiated diamonds, like one of the samples included here (TH 2.163): in such stones, H1b might be detected at temperatures as low as 550 ◦C and an H1c at 600 ◦C, and the temperature when these centers are completely annealed out are significantly higher: for this sample, at 1500 ◦C, the H1b absorption decreased by a factor of 62, while the weak H1c absorption increased by a factor of 3; hence the annealing temperature is >1500 ◦C. All this said, this data might simply mean that these brown diamonds that exhibit H1b and H1c had been exposed naturally to temperatures between 1050 and 1100 ◦C during and after plastic deformation. It must be added that, while in artificially irradiated diamonds with the disappearance of H1b and H1c an array of new absorptions may be created, no new absorptions whatsoever appear in the spectra of brown diamonds with naturally occurring H1b and H1c. Their spectra—from the UV to the NIR, absorption and luminescence—remain essentially unchanged with annealing up to 1500 ◦C, with some minor exceptions such as the NIR features covered here and some PL features discussed later in this paper. It seems clear from this data that the H1b and H1c defects in natural DR brown diamonds is the same defect as the one induced by irradiation and annealing, but that it is simply induced by deformation-related vacancy defects that combine with aggregated nitrogen through natural annealing. All observations made here also allow it to question the direct correlation between the 595 nm defect and H1b/H1c. The H1b/H1c defect in irradiated/annealed diamonds might Minerals 2020, 10, x FOR PEER REVIEW 15 of 37 of 3.5 after two hours of heating at 1500 °C while the 4063 cm−1 peak disappeared; at 77 K, the 4063 cm−1 AC was barely detectable or undetectable and the 4092 cm−1 AC was undetectable. These diamonds also exhibited weak but clearly visible H1b and H1c absorptions prior to the annealing, and both annealed out at 1500 °C (Figure 10). To determine the precise annealing behavior of the ACs in diamonds showing both the 4167 cm−1 and 4063 cm−1 ACs, sample TH 2.330 was heated from 950 to 1350 °C in 100 °C steps and from 1350 °C to 1500 °C in 50 °C steps, for two hours at every step. This treatment has shown that neither the AC nor H1b and H1c were affected up to a temperature of 1050 °C; from 1150 °C onwards the 4063 cm−1 AC got weaker, until it disappeared at 1500 °C, while the 4167 cm−1 AC increased, until it was strongest at 1500 °C. H1b and H1c started to increase from 1150 °C to 1250 °C, when their intensity was found to be 4 to 5 times that of their intensity in the unheated state; then both decreased until they were both undetectable at exactly 1500 °C. Hence, it was shown that the temperature when the 4063 cm−1 AC and H1b H1c completely anneal out is identical. The exact annealing behavior appears to correlate to when the 4063 cm−1 AC starts to decrease in intensity, and H1b and H1c increase significantly; the H1b and H1c defects in these brown diamonds could be interpreted as a side product of the transformation of the 4063 cm−1 AC into the 4167 cm−1 and/or 4111 cm−1 AC. This interpretation is supported by the fact that the H1b and H1c absorptions in the brown diamonds do not increase in intensity before 1150 °C. In the artificially irradiated diamonds of type I studied for various research projects by our laboratory, the common behavior of H1b and H1c is that they are detected between 600 and 750 °C and that these increase to different temperatures, somewhere between 1100 and 1200 °C, when they typically start to decrease and anneal out between 1400 °C and 1500 °C. The situation looks somewhat different for very heavily irradiatedMinerals 2020 diamonds,, 10, 903 like one of the samples included here (TH 2.163): in such stones, H1b might17 of be 38 detected at temperatures as low as 550 °C and an H1c at 600 °C, and the temperature when these centers are completely annealed out are significantly higher: for this sample, at 1500 °C, the H1b absorptionsimply be standalonedecreased by defects a factor resulting of 62, fromwhile irradiation-induced the weak H1c absorption multi-vacancies increased combining by a factor with of 3; A henceand B the aggregates annealing upon temperature annealing, is hence >1500 very°C. similar to what is suggested here for brown diamonds.

−1 1 FigureFigure 10. 10. TheThe NIR NIR spectra spectra (at (at 77 77K) K)of ofa diamond a diamond exhibiting exhibiting both both the theDAC DAC (4167/4063 (4167/4063 cm cm) as− well) as aswell H1b/H1c as H1b before/H1c before (a/a*) and (a/a*) after and annealing after annealing to 1500 to°C 1500for 2h◦C (b/b*). for 2h It (bis /obviousb*). Itis that obvious the 4063/4092 that the −1 1 −1 1 cm4063 /component4092 cm− componentis annealed isout annealed while the out 4167 while cm the ZPL 4167 increases cm− ZPL distinctly increases after distinctly the annealing after the −1 1 experiment.annealing experiment. H1b and H1bH1c andare H1calso areanneal alsoed annealed out under out underthese theseconditions. conditions. The The4200 4200 cm cm band− band is apparentlyis apparently unaffected. unaﬀected. The The spectra spectra were were shifted shifted vertically vertically for for clarity. clarity.

When annealing a type Ib DR brown diamond with distinct ACs to the higher temperatures of 1 1 1850 ◦C we found the same results as described above concerning the ACs at 4063 cm− , 4167 cm− , 1 1 1 4093 cm− and 4111 cm− ; no consistent annealing behavior for the 3462 cm− AC could be determined, since it appears to behave randomly, intensifying in one sample while in a another one it decreased. 1 What seems clear is that the band centered at 2790 cm− can be induced by treatment at 1500 ◦C to 1850 ◦C, since it was usually detectable in heated stones, but was also found in the spectra of several untreated samples. 1 In diamonds with only the 4167 cm− AC detectable, annealing to 1500 ◦C did not affect the AC 1 at all, and neither did heating to 1850 ◦C. In type Ib diamonds with the 4111 cm− AC, annealing 1 for two hours at 1500 ◦C caused a reduction in the intensity of the 4111 cm− ZPL by about 30%, 1 1 while if a small 4167 cm− AC was present the 4111 cm− AC was not affected at all; in type Ib 1 1 1 DR brown diamonds with more complex ACs composed of 4063 cm− , 4111 cm− , and 4167 cm− , 1 the 4112 cm− AC remained unaffected even at temperatures as high as 1850 ◦C. In sample TH 2.52, 1 1 the type Ib/IaA sample with the IR spectrum dominated by 4240 cm− /3462cm− bands at RT and 1 1 strong 4111, 4092 cm− , and 4063 cm− ACs only after photoexcitation or at LNT, annealing at 950 ◦C to 1 1050 ◦C caused a decrease of the 3462 cm− absorption by approximately 20%; the other ACs remained 1 unaffected, and so was H1b. At 1500 ◦C, the 4063 cm− AC annealed out and so did the associated 1 1 1 1 4092 cm− AC, a new AC absorption at 4165 cm− appeared and both the 4111 cm− and 4136 cm− absorptions increased. 1 These results show that the 4063 cm− AC is clearly the least stable of all the ACs since it is affected by heating as low as 1150 ◦C—distinctly lower than the value of 1700 ◦C published earlier [22]—and 1 1 that the 4167 cm− and the 4111 cm− ACs are the most stable of all the ACs as they remain essentially Minerals 2020, 10, 903 18 of 38

unaffected by heating to 1850 ◦C, except for likely some charge-related modifications occurring for the 1 1 4111 cm− AC. Since annealing out the 4063 cm− AC results in a distinct increase or formation of the 1 1 1 4167 cm− AC and when the 4111 cm− AC is present also in an increase in the 4111 cm− AC and the 1 1 1 associated 4136 cm− AC; it seems reasonable to suggest that the 4167 cm− and 4111 cm− AC form 1 1 out of the 4063 cm− AC, hence the 4063 cm− AC appears to be a precursor of the those ACs. HPHT treatment at 2000 to 2250 ◦C performed on selected type I DR diamonds has shown that all AC absorptions described in this chapter are annealed out under such conditions; the effective annealing temperature is somewhere between 1850 and 2000 ◦C, which is consistent with the earlier published values [22]. A summary of the various ACs, their annealing behavior and their possible interpretations can be found in Table3.

Table 3. The ZPL positions of the ACs, their annealing behavior in type I DR brown diamonds, and their possible interpretations.

ZPL LNT ZPL RT Annealing Behavior Possible Interpretation Multi-vacancy defect 4200 4200 Anneals out at >1850 C ◦ involving the B center 1 Without 4063 cm− AC: anneals out at >1850 ◦C. With 4063 cm 1 AC: increases from Multi-vacancy defect 4167 4165 − 1150 ◦C to 1500 ◦C while involving the A center 1 4063 cm− AC decreases, then anneals out at >1850 ◦C Likely different charge state of 4136 - Anneals out at >1850 ◦C one of the defects causing the ACs at 4167/4092/4063 When dominant it reduces by 30% at 1500 C, anneals out at ◦ Likely different charge state of >1850 C. When accompanied 4111 4112 ◦ one of the defects causing the by 4063 cm 1 AC then − ACs at 4167/4092/4063 unaffected until it anneals out at >1850 ◦C Decreases from 1450 C, 4092 - ◦ Multi-vacancy defect anneals out at 1500 ◦C Multi-vacancy Decreases from 1150 ◦C, defect—precursor defect of the 4063 4065 1 anneals out at 1500 ◦C 4167 cm− AC and the 1 4111 cm− AC Multi-vacancy defect n/a 3520 Anneals out at >1850 C ◦ involving the A center Likely different charge state of n/a 3462 Anneals out at >1850 ◦C one of the defects causing the ACs at 4167/4092/4063 Likely different charge states n/a 2915, 2880, 2790 Anneal out at >1850 ◦C of one of the defects causing the ACs at 4167/4092/4063 Multi-vacancy defect n/a 2760 Anneals out at >1850 C ◦ involving the A center

The Behavior of the Amber Centers upon Electron Irradiation and Annealing up to 1050 ◦C Irradiation of type I DR brown diamonds with ACs by 2 to 10 MeV electrons and annealing up to 1050 ◦C has shown that the 4167/4063 ACs decrease in intensity in type Ia diamonds by simple electron irradiation while the 4167/4111/4063 ACs increase in most type Ib diamonds; in some type Ib samples either the ACs remained unchanged or appeared to decrease very slightly, particularly stones in which 1 only the 4167 cm− AC was detected. Annealing up to 1050 ◦C—the maximum temperature before some of the ACs are aﬀected by the heat in untreated diamonds—continuously restored the intensity Minerals 2020, 10, 903 19 of 38 of the ACs to their original level in type Ia diamonds, and in Ib diamonds the ACs either returned to their original level or stayed at an intermediate level. It must be stressed out that no new ACs that were absent prior to irradiation were formed, only the ones present were modiﬁed.

Amber Centers induced by Treatment The hypothesis that the different amber centers are basically all closely related complex vacancy based defects generally associated with nitrogen and that there appears to be a direct link between the amber center formation and the formation of natural H1b and H1c defects in type I DR brown diamonds must lead to the question whether it could be possible to create the amber centers by an irradiation treatment process. Having seen that regular commercial irradiation by relatively low doses of electrons had an influence on already existing amber centers, but that it did not induce new amber centers led us to conclude that possibly a much higher vacancy density would be needed to create an amber center-like defect. To verify this idea, a series of originally colorless type Ia diamonds—all belonging to the cape series—have been irradiated by a very high dose of neutrons (1.8 x 1017 neutrons/cm2) and in consequence turned opaque black. The diamonds were then annealed at different temperatures up to 2000 ◦C and the effects on their color and spectra were analyzed, with a particular focus on the IR and NIR spectra. To our positive surprise, the results of these treatment experiments have shown that heavy neutron irradiation followed by annealing indeed created absorptions that with high probability are identical 1 to the ACs. After annealing at 1000 ◦C, a weak peak at 4063 cm− was created and after 1500 ◦C 1 1 1 weak absorptions at 4167 cm− and 4111 cm− were detected, associated by a band at 3462 cm− and a 1 1 very weak band at 2915 cm− (Figure 11, traces b and d). The reduction of the 4063 cm− absorption 1 while the 4167 cm− band was formed at 1500 ◦C is identical to what we have seen happening when 1 diamonds with a 4063 cm− AC were annealed to 1500 ◦C. 1 After HPHT treatment at 1850 ◦C a much stronger 3462 cm− band was formed and at the same 1 time the band at 2915 cm− increased strongly as well (Figure 11, trace f). Photoexcitation experiments have confirmed that the absorptions behave the same as the ACs in DR brown diamonds, with the 1 1 1 3462 cm− band decreasing while the 4167 cm− and the 4111 cm− bands increased (Figure 11, traces c and e). Furthermore when the NIR spectrum recorded at 77 K was analyzed, a distinct sharpening 1 1 1 and associated increase was noticed for the 4167 cm− and 4111 cm− absorptions while the 4063 cm− feature that could not be resolved at RT was detectable again (Figure 11, trace g). The shift between RT and LNT of these three absorptions was identical to the one observed for the ACs in DR brown diamonds. When diamonds with these treatment-induced absorptions were treated by HPHT at 2000 ◦C, all the described features were annealed out, which—again—is consistent with the behavior of the ACs. High-resolution IR spectra of the treated diamonds have shown that, even after 2 h at only 1500 ◦C, 4 ppm of C centers were detectable and that 8 ppm of the C center were formed after annealing at 1850 ◦C for 5 min. This explains why many of the AC absorptions that were created by heavy neutron irradiation and annealing at 1500 and 1850 ◦C are identical to the ACs that only occur in the spectra of C center containing natural, untreated DR brown diamonds. The dissociation of the aggregated nitrogen into single nitrogen at such a “low” temperature—normally the C center cannot be detected in the IR spectra of HPHT-treated type Ia diamonds with treatment temperatures below 2100 ◦C—is clearly related to the heavy neutron irradiation the diamonds were exposed to and the associated density of induced vacancies. Vacancies are known to enhance the aggregation—and dissociation—of nitrogen since the vacancies create new diffusion channels within the diamond lattice [23]. In conclusion, the absorptions formed by heavy neutron irradiation and annealing correspond to the naturally occurring ACs in position, shape, photoexcitation behavior, low temperature behavior, and stability to high temperature. It seems reasonable to suggest that the absorptions created are indeed treatment-induced ACs. Minerals 2020, 10, 903 20 of 38

Minerals 2020, 10, x FOR PEER REVIEW 19 of 37

FigureFigure 11. The 11. IRThe/NIR IR/NIR spectra spectra showing showing the evolutionthe evolution of anof originallyan originally colorless colorless type type IaA IaA>B>B “cape “cape series” diamondseries” before diamond neutron before treatment neutron trea (tracetment a), (trace after a), heavy after heavy neutron neutron irradiation irradiation and and annealing annealing for for 2h at 2h at 1000 °C (trace b), after 2h at 1500 °C (trace d), after 2h at 1500 °C with photoexcitation (trace c), 1000 ◦C (trace b), after 2h at 1500 ◦C (trace d), after 2h at 1500 ◦C with photoexcitation (trace c), after after 1850 °C for 5 min (trace f) after 1850 °C for 5 min with photoexcitation (trace e), and, finally, the 1850 C for 5 min (trace f) after 1850 C for 5 min with photoexcitation (trace e), and, finally, the low ◦low temperature NIR spectrum after◦ 1850 °C for 5 min (trace g). At 1000 °C, a weak 4065 cm−1 temperature NIR spectrum after 1850 C for 5 min (trace g). At 1000 C, a weak 4065 cm 1 absorption absorption was formed, After 1500 °C,◦ weak 4165 and 4112 cm−1 absorptions◦ were apparent− plus the 1 was formed,absorption After at 15003462◦ C,cm weak−1 appeared 4165 and and 4112 a cmhint− ofabsorptions the 2915 werecm−1 apparentabsorption plus was the detectable; absorption at 1 1 3462 cmphotoexcitation− appeared caused and a the hint 3462 of the cm− 29151 band cm to− decreaseabsorption while was the 4165 detectable; cm−1 peak photoexcitation increased. After caused 1850 the 1 1 3462°C, cm −theband spectral to decrease features detected while the after 4165 annealin cm− peakg at 1500 increased. °C became After distinctly 1850 ◦C, stronger, the spectral but their features detectedbehavior after remained annealing identical. at 1500 ◦TheC became spectrum distinctly recorded stronger, with the butdiamond their behaviorat 77 K shown remained in trace identical. g The spectrumdemonstrates recorded the distinct with the increase, diamond sharpening at 77 K shown and peak in trace shifting g demonstrates of the three the peaks distinct at increase,low sharpeningtemperature. and peakThe peak shifting shapes, of thepositions, three peaksphotoe atxcitation, low temperature. and low temperature The peak behavior shapes, are positions, all photoexcitation,identical to what and has low been temperature described for behavior the ACs, are and all hence identical it appears to what highly has likely been that described what was for the ACs,created and hence are indeed it appears treatment-in highlyduced likely ACs. that The what spectra was were created shifted are vertically indeed for treatment-induced clarity. ACs. The spectra were shifted vertically for clarity. UV-Vis-NIR Spectroscopy UV-Vis-NIRThe Spectroscopy UV-Vis-NIR spectra of all the studied samples showed the continuum absorption increasing steadily from the NIR to the UV, which is so typical of brown diamonds, both type Ia and Ib (Figure The UV-Vis-NIR spectra of all the studied samples showed the continuum absorption increasing 12). Overlaying the continuum many of the spectra exhibited a relatively weak broad band centered steadily from the NIR to the UV, which is so typical of brown diamonds, both type Ia and Ib (Figure 12). at 560 nm, which corresponds to the broad band responsible for the pink color in diamonds, Overlayingcombined the with continuum a band centered many of at the about spectra 390 nm exhibited and that a consists relatively of two weak components broad band in type centered Ia at 560 nm,diamonds, which namely corresponds at 394 and to the 386 broad nm (Figure band 12, responsible trace c). Most for commonly, the pink color a weak in N3 diamonds, and a weak combined H3 with aabsorption band centered was observed at about in 390 the nm spectra and thatof the consists type Ia of brown two componentsdiamonds (Figure in type 12, Ia trace diamonds, b and c)— namely at 394being and usually 386 nm the (Figure responsible 12, trace defects c). for Most the commonly,blue fluorescence a weak (N3) N3 and and in amost weak cases H3 for absorption the green was observedfluorescence in thespectra (H3). Besides of the these, type a Ia handful brown of diamonds weak absorption (Figure feat 12,ures trace were b and found, c)—being none of usuallywhich the responsiblehad any defectssignificant for influence the blue on fluorescence the color. Only (N3) certain and inhydrogen-rich most cases type for theIaB greenDR brown fluorescence diamonds (H3). Besidesshowed these, significantly a handful ofdifferent weak absorptionspectra, in featureswhich the were continuum found, nonewas overlaid of which by had a broad any significantband centered at 580 nm, a ZPL at 500.2 nm with broad vibronic bands similar to H3, and a ZPL at 406.4 influence on the color. Only certain hydrogen-rich type IaB DR brown diamonds showed significantly nm with a broad vibronic structure similar to N3 (Figure 12, trace e). different spectra, in which the continuum was overlaid by a broad band centered at 580 nm, a ZPL Truly brown type Ib diamonds, being the rarest of all brown diamonds, also showed the well- at 500.2described nm with vacancy-cluster broad vibronic related bands continuum similar absorpti to H3,on and in their a ZPL UV-Vis-NIR at 406.4 nm spectra, with but a broad instead vibronic of structurethe spectral similar features to N3 (Figuredescribed 12 above, trace they e). show a distinct NV− center absorption with its ZPL at 637.0 Trulynm (Figure brown 12, trace type a). Ib diamonds, being the rarest of all brown diamonds, also showed the well-described vacancy-cluster related continuum absorption in their UV-Vis-NIR spectra, but instead of the spectral features described above they show a distinct NV− center absorption with its ZPL at 637.0 nm (Figure 12, trace a). Minerals 2020, 10, 903 21 of 38 Minerals 2020, 10, x FOR PEER REVIEW 20 of 37

Minerals 2020, 10, x FOR PEER REVIEW 20 of 37

Figure 12. The rangerange of of UV-Vis-NIR UV-Vis-NIR spectra spectra of typeof type I brown I brown diamonds diamonds with awith plastic a plastic deformation-related deformation-

relatedcolor is color shown is inshown this graph. in this The graph. brown The color brown itself colo originatesr itself originates in all cases in mainly all cases from mainly the continuum from the Figure 12. The range of UV-Vis-NIR spectra of type I brown diamonds with a plastic deformation- continuumabsorption thatabsorption is relatively that flat is relatively and steadily flat increasing and steadily from increasing the NIR to UV.Sincefrom the theNIR indexed to UV. overlaying Since the related color is shown in this graph. The brown color itself originates in all cases mainly from the indexedabsorptions overlaying are all comparativelyabsorptions are weak, all comparatively they have only weak, a limited they have influence only a limited on the color.influence A notable on the continuum absorption that is relatively flat and steadily increasing from the NIR to UV. Since the color. A notable exception is the 560 nm band, which causes a pink hue added to the brown. The exceptionindexed is theoverlaying 560 nm absorptions band, which are causesall comparatively a pink hue weak, added they to have the only brown. a limited The spectrainfluence were on the shifted spectra were shifted vertically for clarity. verticallycolor. forA notable clarity. exception is the 560 nm band, which causes a pink hue added to the brown. The spectra were shifted vertically for clarity. This study confirmed confirmed that the lightness/darkness lightness/darkness of the brown color directly correlate with the steepnessThis of of the study absorption confirmed continuum, that the lightness/darkness with with an an important important of the brown point point color being being directly that that the correlate continuum with the has has a relativelysteepness consistent of the inclinationabsorption continuum, over the visible with anspectral important range point (Figure being 1313). that). If If thethe the continuumcontinuum continuum has becomes becomes a significantlysignificantlyrelatively steeper consistent somewhere inclination between over the visible 550 and spectral 6 60000 nm, range the (Figure color becomes 13). If the more continuum saturated becomes and will shift towardssignificantly orange steeper or somewhereyellow. between 550 and 600 nm, the color becomes more saturated and will shift towards orange or yellow.

FigureFigure 13. The 13. The UV-Vis-NIR UV-Vis-NIR spectra spectra of of stones stones fromfrom the co colorlor suite suite shown shown in Figure in Figure 7 demonstrate7 demonstrate that that the darkness of the brown diamonds is represented by the steepness of the absorption continuum. Figurethe darkness 13. The of UV-Vis-NIR the brown diamondsspectra of stones is represented from the bycolor the suite steepness shown of in the Figure absorption 7 demonstrate continuum. that The spectra shown are the most common type of UV-Vis-NIR spectra, with a characteristic The spectra shown are the most common type of UV-Vis-NIR spectra, with a characteristic continuum, the darknesscontinuum, of weakthe brown N3 and diamonds H3 absorption is represente and often ad detectable by the steepness 560 nm band. of theThe absorptionspectra were continuum.shifted weak N3 and H3 absorption and often a detectable 560 nm band. The spectra were shifted vertically The verticallyspectra shownfor clarity. are the most common type of UV-Vis-NIR spectra, with a characteristic continuum,for clarity. weak N3 and H3 absorption and often a detectable 560 nm band. The spectra were shifted vertically for clarity.

Minerals 2020, 10, 903 22 of 38

PhotoluminescenceMinerals 2020, 10, x FOR Spectroscopy PEER REVIEW 21 of 37

ThePhotoluminescence DR type Ia brown Spectroscopy diamonds showed a range of PL spectra, of which some were very common and othersThe much DR rarer.type Ia The brown most diamonds common showed type of a PL range data of is shownPL spectra, in Figures of which 14 someand 15 were, even very though theycommon represent and rather others extreme much rarer. cases The for most certain common defects. type The of PL most data commonly is shown in dominating Figures 14 and PL 15, defects detectedeven inthough such they diamonds represent are rather the H3 extreme center cases with for its certain ZPL atdefects. 503.2 The nm, most the 617.6commonly/626.3 dominating nm defect and its associatedPL defects vibronicdetected in band such at diamonds 720 nm are [24 ],the the H3 490.8 center nm with defect, its ZPL the at 503.2 576.0 nm, nm the defect—often 617.6/626.3 nm with a weakerdefect peak and at its 575.5 associated nm—the vibronic 612.5 band nm defect,at 720 nm N3 [24], (ZPL the 415.2 490.8 nm),nm defect, and H4 the (ZPL 576.0 495.9nm defect—often nm). Very often detectedwith buta weaker rarely peak intense at 575.5 are defectsnm—the with 612.5 the nm ZPLs defect, at N3 406.1 (ZPL nm, 415.2 412.1 nm), nm, and 422.8, H4 (ZPL 494.3 495.9 nm, nm). 500.1 nm, Very often detected but rarely intense are defects with the ZPLs at 406.1 nm, 412.1 nm, 422.8, 494.3 535.9 nm, 537.5 nm, 578.1 nm, 578.9 nm, NV− (ZPL 637.0), 710.3 nm, 768.0 nm, 787.1 nm, 793.6 nm, nm, 500.1 nm, 535.9 nm, 537.5 nm, 578.1 nm, 578.9 nm, NV− (ZPL 637.0), 710.3 nm, 768.0 nm, 787.1 and 925.4/926.2 nm doublet. Only detectable in diamonds with low nitrogen content were peaks at nm, 793.6 nm, and 925.4/926.2 nm doublet. Only detectable in diamonds with low nitrogen content 566.0, 566.7, and 567.6 nm and distinct GR1 PL (Figure 16). were peaks at 566.0, 566.7, and 567.6 nm and distinct GR1 PL (Figure 16).

FigureFigure 14. 14.The The low low temperature temperature PLPL spectraspectra of a pink pinkishish brown brown type type IaA>>B IaA>> diamondB diamond with witha a deformation-relateddeformation-related color, color, recorded recorded with with four four didiffeﬀerentrent lasers. Besides Besides the the omnipresent omnipresent H3 defect H3 defect and and the 490.8 nm defect, the most characteristic band seen in most of such diamonds is the 617.6/626.3 nm the 490.8 nm defect, the most characteristic band seen in most of such diamonds is the 617.6/626.3 nm defect with its associated broad vibronic band centered approximately at 720 nm. This defect is defect with its associated broad vibronic band centered approximately at 720 nm. This defect is responsible for the pink fluorescence seen in the diamond under DFI 340 nm excitation (photo inset, responsible for the pink ﬂuorescence seen in the diamond under DFI 340 nm excitation (photo inset, right). The spectra were shifted vertically for clarity. right). The spectra were shifted vertically for clarity. The 617.6/626.3 nm defect seems to be unique to DR type Ia brown diamonds with appreciable Thenitrogen 617.6 content,/626.3 nmsince defect it was seemsabsent toin the be uniquelow nitrogen to DR DR type brown Ia browndiamonds diamonds and also within the appreciable type II nitrogenDR brown content, diamonds. since it wasFurther, absent in the in experience the low nitrogen of the authors, DR brown this defect diamonds is unknown and also in colorless in the type II DR brownand any diamonds. otherwise-colored Further, diamonds. in the experience De Weerdt of and the Collins authors, have this concluded defect is in unknown their 2007 in paper colorless [24] and any otherwise-coloredthat the 617.6 and 626.3 diamonds. nm lines De were Weerdt not ZPLs, and bu Collinst vibronic have bands, concluded based on in their observation 2007 paper that [24] that the 617.6the peaks and 626.3did not nm weaken lines weredistinctly not at ZPLs, higher but temperature. vibronic bands, In our based experiments on their it was observation obvious that that the peaksthe did two not peaks weaken (and distinctly the other atassociated higher temperature.vibronic peaks) In started our experiments to decrease itvery was rapidly obvious when that the two peaksincreasing (and the the temperature other associated and that vibronic they were peaks) unde startedtectable toat RT, decrease while the very 720 rapidly nm band when remained increasing unchanged (Figure 17). Further, these peaks showed variable intensity depending on the laser the temperature and that they were undetectable at RT, while the 720 nm band remained unchanged excitation (Figure 14), a behavior typical of certain ZPLs, such as the S1, S2, and S3 centers. Both the (Figureshape 17). of Further, the two these peaks peaks and their showed behavior variable have intensity led to our depending conclusion on that the the laser two excitation peaks represent (Figure 14), a behaviorthe ZPLs typical of the of 720 certain nm vibronic ZPLs, such band. as theThe S1,617.6/626.3 S2, and S3nmcenters. center appears Both the to shapebe the ofonly the defect two peaks and theirdetected behavior by PL spectroscopy have led to that our is conclusion unique to DR that type the Ia twobrown peaks diamonds. represent Many the of the ZPLs other of defects the 720 nm vibroniclisted band. above The are 617.6typical/626.3 for such nm brown center diamonds, appearsto ye bet they the are only not defect unique detected to them byand PL can—in spectroscopy the that isexperience unique toof DRthe authors—be type Ia brown found diamonds. in other diamon Manyds of that the have other undergone defects listed plastic above deformation, are typical in for suchany brown color, diamonds, including yetcolorless. they are not unique to them and can—in the experience of the authors—be found in other diamonds that have undergone plastic deformation, in any color, including colorless. Minerals 2020, 10, 903 23 of 38

Minerals 2020, 10, x FOR PEER REVIEW 22 of 37

Minerals 2020, 10, x FOR PEER REVIEW 22 of 37 340 nm

340 nm

FigureFigure 15. The 15. lowThe temperaturelow temperature PL spectraPL spectra of aof brown a brow typen type IaB >IaB>AA diamond diamond with with a deformation-relateda deformation- related color, recorded with three different laser excitations. Interesting in this diamond is that the color, recorded with three different laser excitations. Interesting in this diamond is that the blue Figureblue component 15. The low of the temperature fluorescence PL is spectra not mainly of a ca browusedn by type the IaB>AN3 center, diamond but by with the 406.1a deformation- nm defect. component of the fluorescence is not mainly caused by the N3 center, but by the 406.1 nm defect. relatedThis defect color, is veryrecorded common with in three DR brown different diamonds, laser ex citations.but rarely Interesting as intense asin sthiseen diamondhere. The otheris that very the This defect is very common in DR brown diamonds, but rarely as intense as seen here. The other very bluetypical component defects for of DRthe brownfluorescence diamonds is not are mainly 490.8, ca H4,used H3, by 535.9, the N3 576.0, center, 612.5 but andby the the 406.1 617.6/626.1 nm defect. nm typicalThiscenter. defects defect While is for veryin DR most common brown brown diamondsin diamonds DR brown the are diamonds, H3 490.8, defect H4, bu is tresponsible H3, rarely 535.9, as intense for 576.0, the as commonly 612.5 seen here. and observed theThe 617.6other green /very626.1 nm center.typicalfluorescence, While defects in most in for this DR brown diamond brown diamonds thediamonds 490.8 thenm are H3defect 490.8, defect is H4, dominantly is H3, responsible 535.9, responsible 576.0, for 612.5 the for commonlyand the thegreen 617.6/626.1 fluorescence observed nm green fluorescence,center.seen in Whilethe in photo this in most diamond (inset, brown right). the diamonds The 490.8 spec nm thetra defect H3were defe shifted isct dominantly is responsible vertically responsible for for clarity. the commonly for the observed green fluorescence green seenfluorescence, in the photo in (inset, this diamond right). Thethe 490.8 spectra nm defect were shiftedis dominantly vertically responsible for clarity. for the green fluorescence seen in the photo (inset, right). The spectra were shifted vertically for clarity. 220

220

Figure 16. The low temperature PL spectra of a brown-yellow low nitrogen type IaAB diamond with a deformation-related color, recorded with three different laser excitations. While many of the defects FigureFigureobserved 16. 16.The are The lowthe low same temperature temperature as the ones PL PL spectraobserved spectra of ain of brown-ye higher a brown-yellow nitrogenllow low containi nitrogen low nitrogenng type brown IaAB typediamonds, diamond IaAB some with diamond withadefects a deformation-related deformation-related are notably absent color, color,with recorded the recorded most with noteworthy three with different three being di laserﬀ theerent 617.6/626.1 excitations. laser excitations. nm While center. many In While this of the diamond,many defects of the the H3 and N3 defects are responsible for the green and blue fluorescence seen in the photo (inset, defectsobserved observed are the are same the same as the as ones the observed ones observed in higher in highernitrogen nitrogen containing containing brown diamonds, brown diamonds,some defectsright). This are notably type of absentspectrum with is the similar most to noteworthy what can be being seen the for 617.6/626.1 many type nm IIa center. diamonds. In this The diamond, spectra some defects are notably absent with the most noteworthy being the 617.6/626.1 nm center. In this thewere H3 shifted and N3 vertically defects forare clarity. responsible for the green and blue fluorescence seen in the photo (inset, diamond, the H3 and N3 defects are responsible for the green and blue ﬂuorescence seen in the right). This type of spectrum is similar to what can be seen for many type IIa diamonds. The spectra photo were (inset, shifted right). vertically This typefor clarity. of spectrum is similar to what can be seen for many type IIa diamonds. The spectra were shifted vertically for clarity.

Minerals 2020, 10, 903 24 of 38 Minerals 2020, 10, x FOR PEER REVIEW 23 of 37

Figure 17.17. The 402 nmnm PLPL spectraspectra ofof thethe pinkishpinkish brownbrown typetype IaAIaA>>B>>B diamond shown in Figure 14,14, −1 recordedrecorded atat variousvarious temperaturestemperatures fromfrom roomroom temperaturetemperature toto 19696 °CC (LNT) and normalized to thethe − ◦ intensityintensity ofof thethe 720720 nmnm band.band. The shape and behavior of the 617.6617.6/626.3/626.3 nm peakspeaks are indicative that thesethese featuresfeatures representrepresent zerozero phononphonon lineslines andand notnot vibronicvibronic components.components. TheThe correlationcorrelation ofof thesethese twotwo ZPLs withwith thethe broadbroad 720 720 nm nm band band led led us us to to suggest suggest that that they they are are the the ZPLs ZPLs of theof the defect defect responsible responsible for thefor 720the nm720 band,nm band, hence hence the 720 the nm 720 band nm is band the vibronic is the vibronic component. component. The spectra The were spectra shifted were vertically shifted forvertically clarity. for clarity.

The PL spectra spectra of of the the type type Ib IbDR DR brown brown diamonds diamonds were were very very different diﬀerent from from type typeIa stones: Ia stones: their 0 0 − theirspectra spectra were weregenerally generally dominated dominated by NV by NV andand NV NV centers− centers (ZPL’s (ZPL’s at at574.9 574.9 nm nm and and 637.0 nmnm respectively), with distinct H3 (ZPL 503.2 nm) and very weak N3 (ZPL 415.2) center PL (Figure 18);18); a weakweak peakpeak atat 565.7 565.7 nm nm was was present present in in some some of theof the spectra spectra as well,as well, which which is characteristic is characteristic for the for vast the majorityvast majority of type of type Ib diamonds Ib diamonds [13]. [13]. Most Most of the of type the Ibtype DR Ib brown DR brown diamonds diamonds were were found found to be to pure be typepure Ibtype without Ib without any aggregated any aggregated nitrogen nitrogen detectable de viatectable IR spectroscopy; via IR spectroscopy; the high vacancythe high density vacancy in suchdensity diamonds in such combineddiamonds withcombined the C centerswith the results C centers in the results dominant in the NV dominant center PL. NV The center presence PL. The of N3presence and H3 of indicatesN3 and H3 that indicates some aggregated that some aggregated nitrogen must nitrogen be present, must be even present, though even a good though portion a good of theportion A aggregates of the A aggregates are surely boundare surely in thebound H3 centersin the H3 and centers thus not and visible thus not in thevisible IR spectra. in the IR The spectra. very weakThe very N3 PLweak in contrast N3 PL in requires contrast the requires presence the of atpres leastence some of at B least centers, some even B centers, though theeven concentration though the isconcentration certainly extremely is certainly low, andextremely below thelow, detection and below limit the of IR detection spectroscopy. limit Interestingly,of IR spectroscopy. sample THInterestingly, 2.561, a pure sample type IbTH diamond 2.561, a withpure only type 2 ppmIb diamond of C centers, with wasonly characterized 2 ppm of Cby centers, PL spectra was 0 0 − withcharacterized a very dominant by PL H3spectra center with and a only very minor dominant NV and H3 NV center−; furthermore, and only N3minor was NV easily and detected NV ; 1 1 −1 byfurthermore, PL. This stone N3 was was easily shown detected to exhibit by PL. distinct This 4111stone cm was− shownand 4167 to cmexhibit− ACs distinct in its 4111 IR spectrum, cm and which4167 cm already−1 ACs indicatedin its IR spectrum, the presence which ofaggregated already indicate nitrogen.d the Since presence the aggregated of aggregated nitrogen nitrogen. could Since not bethe found aggregated in the IRnitrogen spectra, could it must not be assumedbe found that in allthethe IR aggregatedspectra, it nitrogenmust be wasassumed bound that by the all ACs the andaggregated by the H3 nitrogen defect, was and bound hence by became the ACs undetectable and by the by H3 IR defect, spectroscopy. and hence became undetectable by IR spectroscopy.

Minerals 2020, 10, 903 25 of 38 Minerals 2020, 10, x FOR PEER REVIEW 24 of 37

265

Figure 18.18. The low temperaturetemperature PLPL spectraspectra ofof aa veryvery rare rare brown brown low low nitrogen nitrogen type type Ib Ib diamond diamond with with a deformation-relateda deformation-related color, color, recorded recorded with fourwith difourﬀerent different laser excitations. laser excitations. The spectra The of spectra such diamonds of such 0 0 − 0 arediamonds always dominatedare always by dominated NV and NV by− NVcenters, and combined NV centers, with thecombined H3 defect. with In the this H3 diamond, defect. the In NVthis defectsdiamond, are the responsible NV0 defects for the are red responsible orange ﬂuorescence for the red seen orange in the fluorescence photo (inset, seen right). in Thethe photo spectra (inset, were shiftedright). The vertically spectra for were clarity. shifted vertically for clarity.

The Behavior ofof PLPL DefectsDefects uponupon AnnealingAnnealing While the annealing annealing behavior behavior of of most most PL PL defect defectss of DR of DRbrown brown diamonds diamonds by temperatures by temperatures used usedduring during the HPHT the HPHT treatment treatment (2000–2500 (2000–2500 °C)◦ C)are are rela relativelytively well well known, known, their their behavior behavior at at lower temperaturestemperatures are not wellwell studiedstudied and only somesome data on LPHT (low pressure, high temperature) treatmentstreatments hashas beenbeen publishedpublished forfor typetype IaIa diamondsdiamonds [[22].22]. Therefore, annealingannealing experiments up to 15001500 ◦°CC were performedperformed toto checkcheck thethe stabilitystability ofof thethe PLPL defectsdefects inin DRDR typetype II brownbrown diamonds.diamonds. Of all thethe commoncommon defects detected in this study, onlyonly aa fewfew werewere consistentlyconsistently aaffectedffected inin thethe samplessamples thatthat werewere treated.treated. TheThe least least stable stable defect defect besides besides GR1—which GR1—which anneals anneals out atout 800 at◦ C—is800 °C—is the 612.5 the nm612.5 defect; nm thisdefect; defect this wasdefect either was annealedeither annealed out completely out complete or reducedly or reduced by >95% by >95% at 1500 at ◦1500C. The °C. H4The center H4 center was foundwas found to reduce to reduce in intensity in intensity by about by about 75%, and 75%, finally and finally the 406.2 the nm 406.2 defect nm reduced defect reduced by 40% atby 1500 40%◦ C.at Some1500 °C. of theSome other of the defects other behaved defects irregularly,behaved irregularly, in some cases in some they reducedcases they significantly reduced significantly in other cases in theyother remained cases they una remainedffected; theseunaffected; include these the 422.8include nm the defect, 422.8 the nm 490.8 defect, nm the defect, 490.8 and nm thedefect, NV− andcenter. the InNV type− center. Ib diamonds, In type noneIb diamonds, of the defects none were of the aff ecteddefects by were annealing affected at the by highestannealing temperature. at the highest temperature. DFI Fluorescence Imaging and Spectroscopy DFI FluorescenceUsing a standard Imaging 365 and nm Spectroscopy (LW) and 254 nm (SW) fluorescence lamp many the brown diamonds—particularlyUsing a standard 365 the nm darker (LW) and ones—only 254 nm (SW) exhibited fluorescence a rather lamp weak many fluorescence, the brown being diamonds— typically blue,particularly green, the or orangedarker ones—only in color under exhibited LW and a rather blue, green,weak fluorescence, or yellow under being SW typically when blue, discernible, green, otherwiseor orange in they color appeared under LW practically and blue, inert. green, The or lighter-colored yellow under SW diamonds when discernible, were the most otherwise fluorescent they onesappeared while practically the darkest inert. ones The were lighter-colored the least fluorescent diamonds ones. were This the phenomenon most fluorescent is likely ones related while to the highdarkest density ones ofwere vacancy the least clusters fluoresc presentent ones. in such This stones, phenomenon and an associated is likely related suppression to the high of fluorescence density of invacancy an overly clusters defective present structure. in such stones, and an associated suppression of fluorescence in an overly defectiveUnder structure. the powerful UV excitations of the Mega-DFI system, the diamonds all exhibited distinctUnder fluorescence, the powerful with UV variable excitations colors of and the distribution,Mega-DFI system, depending the diamonds on the excitation all exhibited wavelength. distinct Whenfluorescence, tested with with the variable different colors excitations and distributi available,on, the depending most common on the PL excitation that could wavelength. be seen was When green, blue,tested or with mixes the of different green and excitations blue under available, all excitations the most>240 common nm, while PL the that PL could was always be seen blue was to green, green blueblue, underor mixes the of excitations green and< blue230 nmunder (Figure all excitation 19). Muchs >240 more nm, rarely while a pinkthe PL to was orange always red PLblue under to green the blue under the excitations <230 nm (Figure 19). Much more rarely a pink to orange red PL under the longer excitation wavelengths was observed, while such samples showed a blue PL under the shortest wavelengths (Figure 20).

Minerals 2020, 10, x FOR PEER REVIEW 25 of 37 Minerals 2020, 10, x FOR PEER REVIEW 25 of 37 The 340 nm excitation, which is most comparable to LWUV, excited mostly either green, blue, The 340 nm excitation, which is most comparable to LWUV, excited mostly either green, blue, or pink fluorescence in the type Ia diamonds, often mixed with each other (Figure 19). The causes of or pink fluorescence in the type Ia diamonds, often mixed with each other (Figure 19). The causes of the different visually observed fluorescent colors where verified by the spectrometer that is part of the different visually observed fluorescent colors where verified by the spectrometer that is part of the Mega-DFI system and it was found that green PL in type Ia DR brown diamonds can be caused the Mega-DFI system and it was found that green PL in type Ia DR brown diamonds can be caused by the following defects: H3 (503.2 nm ZPL, by far most common), the 490.8 nm defect (common), H4 by the following defects: H3 (503.2 nm ZPL, by far most common), the 490.8 nm defect (common), H4 (495.9 nm ZPL, rather rare), and S3 (496.6 nm ZPL, unusual, only in cuboid growth sectors). The blue (495.9 nm ZPL, rather rare), and S3 (496.6 nm ZPL, unusual, only in cuboid growth sectors). The blue fluorescence generally thought to be uniquely caused by the N3 center in diamonds can be caused by fluorescence generally thought to be uniquely caused by the N3 center in diamonds can be caused by the N3 center (415.2 nm ZPL, most common), but in some diamonds included in this study it was the N3 center (415.2 nm ZPL, most common), but in some diamonds included in this study it was also caused by the 406.1 nm defect (Figure 15). Theoretically it could also be caused by 412.1 nm and also caused by the 406.1 nm defect (Figure 15). Theoretically it could also be caused by 412.1 nm and 422.8 nm defects, but no example with these features intense enough to visually influence the 422.8 nm defects, but no example with these features intense enough to visually influence the Mineralsobservable2020, 10 PL, 903 were found. The pink to red fluorescence in type Ia DR brown diamonds almost always26 of 38 observable PL were found. The pink to red fluorescence in type Ia DR brown diamonds almost always relates to the 617.6/626.3 nm defect (Figures 14 and 15), but in certain cases an orange to pink relates to the 617.6/626.3 nm defect (Figures 14 and 15), but in certain cases an orange to pink luminescence was caused by a dominant 576.0 nm defect that, whenever dominant, occurred together longerluminescence excitation was wavelengths caused by a was dominant observed, 576.0 while nm suchdefect samples that, whenever showed dominant, a blue PL under occurred the shortesttogether with an unusually intense 535.9 nm defect. wavelengthswith an unusually (Figure intense 20). 535.9 nm defect.

FigureFigure 19. 19.A A lightlight brownbrown typetype IaAIaA>B>B diamond (sample(sample THTH 2.435)2.435) underunder threethree didifferentﬀerent UVUV excitationsexcitations Figure 19. A light brown type IaA>B diamond (sample TH 2.435) under three different UV excitations ofof the the DFI DFI system. system. TheThe imagesimages clearlyclearly showshow thethe defectdefect distributiondistribution alongalong thethe deformation-relateddeformation-relatedslip slip of the DFI system. The images clearly show the defect distribution along the deformation-related slip planesplanes along along (111). (111). planes along (111).

FigureFigure 20.20. AA pinkish pinkish brown brown type type IaA>>B IaA>> Bdiamond diamond (sam (sampleple TH TH 2.440) 2.440) under under three three different different UV Figure 20. A pinkish brown type IaA>>B diamond (sample TH 2.440) under three different UV UVexcitations excitations of the of DFI the system. DFI system. The images The clearly images show clearly the defect show distribution the defect distribution along the deformation- along the excitations of the DFI system. The images clearly show the defect distribution along the deformation- deformation-relatedrelated slip planes along slip planes(111). At along 340 nm, (111). the Atmain 340 defect nm, thevisible main as pink defect PL visible is the as617.6/626.3 pink PL nm is related slip planes along (111). At 340 nm, the main defect visible as pink PL is the 617.6/626.3 nm thedefect; 617.6 the/626.3 green nm H3 defect; PL can the be green seen under H3 PL all can excitations, be seen under and at all 220 excitations, nm the N3 and defect at 220 is dominant. nm the N3 defect; the green H3 PL can be seen under all excitations, and at 220 nm the N3 defect is dominant. defect is dominant. While by far most of the DR brown diamonds were characterized as being of very dominantly While by far most of the DR brown diamonds were characterized as being of very dominantly octahedralThe 340 growth, nm excitation, it was obvious which isfrom most the comparable typical growth to LWUV, patterns—often excited mostly exhibiting either a green, “Mercedes blue, octahedral growth, it was obvious from the typical growth patterns—often exhibiting a “Mercedes orstar” pink pattern—that fluorescence the in thetype type IaA>>B Ia diamonds, nitrogen and often hydrogen-rich mixed with each DR otherbrown (Figure diamonds 19). were The causes of mixed of thestar” di ffpattern—thaterent visually the observed type IaA>>B fluorescent nitrogen colors and hydrogen-rich where verified DR by thebrown spectrometer diamonds thatwere is of part mixed of cuboid-octahedral growth (Figure 21). cuboid-octahedral growth (Figure 21). the Mega-DFIIn the majority system of and type it Ib was DR found brown that diamonds, green PL the in typefluorescence Ia DR brown was orange diamonds to orange-red can be caused from In the majority of type Ib DR brown diamonds, the fluorescence was orange to orange-red from bythe the NV following0 center defects:(574.9 ZPL); H3 (503.2 this nmfluorescence ZPL, by far was most found common), to be the distributed 490.8 nm defectin a much (common), more the NV0 center (574.9 ZPL); this fluorescence was found to be distributed in a much more H4homogenous (495.9 nm manner ZPL, rather than rare), in type and Ia S3 DR (496.6 brown nm diamonds, ZPL, unusual, and there only inwas cuboid hardly growth any particular sectors). homogenous manner than in type Ia DR brown diamonds, and there was hardly any particular Thesectoring blue fluorescence or distribution generally visible (Figure thought 22). to beOne uniquely sample (TH caused 2.561), by thea pure N3 type center Ib diamond in diamonds with can the sectoring or distribution visible (Figure 22). One sample (TH 2.561), a pure type Ib diamond with the belowest caused measurable by the N3 nitrogen center (415.2 content nm of ZPL, all mostdiamonds common), studied but ( in≈2 someppm diamondsof C centers), included surprisingly in this studylowest it measurable was also caused nitrogen by the content 406.1 nmof all defect diamonds (Figure studied15). Theoretically (≈2 ppm of it couldC centers), also be surprisingly caused by

412.1 nm and 422.8 nm defects, but no example with these features intense enough to visually inﬂuence the observable PL were found. The pink to red ﬂuorescence in type Ia DR brown diamonds almost always relates to the 617.6/626.3 nm defect (Figures 14 and 15), but in certain cases an orange to pink luminescence was caused by a dominant 576.0 nm defect that, whenever dominant, occurred together with an unusually intense 535.9 nm defect. While by far most of the DR brown diamonds were characterized as being of very dominantly octahedral growth, it was obvious from the typical growth patterns—often exhibiting a “Mercedes star” pattern—that the type IaA>>B nitrogen and hydrogen-rich DR brown diamonds were of mixed cuboid-octahedral growth (Figure 21). Minerals 2020, 10, 903 27 of 38

In the majority of type Ib DR brown diamonds, the fluorescence was orange to orange-red from the NV0 center (574.9 ZPL); this fluorescence was found to be distributed in a much more homogenous manner than in type Ia DR brown diamonds, and there was hardly any particular sectoring or distributionMinerals 2020, 10 visible, x FOR PEER (Figure REVIEW 22). One sample (TH 2.561), a pure type Ib diamond with the lowest26 of 37 Minerals 2020, 10, x FOR PEER REVIEW 26 of 37 measurable nitrogen content of all diamonds studied ( 2 ppm of C centers), surprisingly exhibited exhibited a purely yellowish green H3 fluorescence under≈ all excitations of the DFI system (Figure aexhibited purely yellowish a purely yellowish green H3 fluorescencegreen H3 fluorescence under all excitationsunder all excitations of the DFI of system the DFI (Figure system 23 ).(Figure With 23). With the H3 center consisting of the A aggregate combined with a vacancy it appears obvious the23). H3With center the H3 consisting center consisting of the A aggregateof the A aggr combinedegate combined with a vacancy with a itvacancy appears it obviousappears thatobvious the that the diamond should contain mainly A aggregates instead of C centers; one would expect the diamondthat the diamond should contain should mainly contain A mainly aggregates A aggregat insteades of instead C centers; of C one centers; would one expect would the vacancyexpect the to vacancy to combine with the C center and result in NV centers and consequently orange to red PL. A combinevacancy to with combine the C centerwith the and C resultcenter in and NV result centers in NV and centers consequently and consequently orange to red orange PL. A to closer red PL. look A closer look at the diamond showed other indications that aggregated nitrogen should be present, atcloser the diamondlook at the showed diamond other showed indications other that indications aggregated that nitrogen aggregated should nitrogen be present, should such be present, a rather such a rather strong1 4167 cm−1 AC. The only way one can explain that absence of aggregated nitrogen strongsuch a rather 4167 cm strong− AC. 4167 The cm only−1 AC. way The one only can way explain one can that explain absence that of absence aggregated of aggregated nitrogen innitrogen the IR in the IR spectrum is that all the A aggregates available were bound to the H3 and the 41671 cm−1 AC spectrumin the IR spectrum is that all is the that A aggregatesall the A aggregates available available were bound were to bound the H3 to and the the H34167 and cmthe− 4167AC cm defects.−1 AC defects. Nonetheless, this does not explain why the vacancies bound to these A aggregates instead of Nonetheless,defects. Nonetheless, this does this not does explain not explain why the why vacancies the vacancies bound bound to these to A these aggregates A aggregates instead instead of the of C the C centers; the energy required to combine the vacancy with an A aggregate may be lower, which centers;the C centers; the energy the energy required required to combine to combine the vacancy the vacancy with an with A aggregate an A aggregate may be may lower, be whichlower, wouldwhich would explain the observed features. explainwould explain the observed the observed features. features.

Figure 21. A DR brown hydrogen- and nitrogen-rich type IaA>>B diamond (sample TH 2.622) under Figure 21. A DR brown hydrogen- andand nitrogen-richnitrogen-rich typetype IaAIaA>>B>>B diamond (sample TH 2.622) under three different UV excitations of the DFI system. The images clearly show the typical “Mercedes-star” threethree didifferentﬀerent UVUV excitationsexcitations ofof thethe DFI system. The images clearly show show the the typical typical “Mercedes-star” “Mercedes-star” pattern visible in mixed cuboid-octahedral growth diamonds. In this diamond, the defects were patternpattern visiblevisible inin mixedmixed cuboid-octahedralcuboid-octahedral growthgrowth diamonds.diamonds. In this diamond, the defects were distributed along the cuboid sectors and along the DR slip planes. Along the cuboid sectors, the S3 distributed along the cuboid sectors and along thethe DRDR slipslip planes.planes. Along the cuboid sectors, the S3S3 defect can be be detected, detected, while while within within the the deformation- deformation-relatedrelated slip slip planes planes the the H3 H3 defect defect is found. is found. At defect can be detected, while within the deformation-related slip planes the H3 defect is found. At At340 340 nm, nm, the the main main defect defect visible visible as gr aseen green PL PLis the is the H3 H3 defect defect while while unde underr 265 265 nm nm excitation excitation it is it the is the S3 340 nm, the main defect visible as green PL is the H3 defect while under 265 nm excitation it is the S3 S3defect; defect; at 220 at 220 nm, nm, the the N3 N3 defect defect combined combined with with the the S3 S3center center are are dominant. dominant. defect; at 220 nm, the N3 defect combined with the S3 center are dominant.

Figure 22.22. A pure type Ib brownbrown diamonddiamond (sample TH 2.51)2.51) under three didifferentﬀerent UVUV excitationsexcitations ofof Figure 22. A pure type Ib0 brown diamond (sample TH 2.51) under three different UV excitations of thethe DFIDFI system.system. The NV0 centercenter is responsible for the luminescenceluminescence under all excitations,excitations, and thethe the DFI system. The NV0 center is responsible for the luminescence under all excitations, and the distribution of the defectsdefects isis muchmuch lessless obviousobvious thanthan inin otherother deformation-relateddeformation-related samples.samples. Photos: distribution of the defects is much less obvious than in other deformation-related samples. Photos: Dr. ThomasThomas Hainschwang.Hainschwang. Dr. Thomas Hainschwang.

Minerals 2020, 10, 903 28 of 38 MineralsMinerals 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 2727 ofof 3737

FigureFigure 23.23. AA pure pure type type Ib Ib brown brown diamond diamond (sample (sample TH TH 2.56 2.561)2.561)1) under under three three different didifferentfferent UVUV excitationsexcitations ofof thethe DFIDFI system.system. The TheThe yellowish yellowishyellowish green greengreen PL PL is is caused caused by by the the H3 H3 defect, defect, even even though though according according to to thethe IRIR spectrumspectrum thethe stonestone diddid notnot containcontain measurablemeasurable AA aggregateaggregate nitrogen.nitrogen.nitrogen. 3.1.2. Type II Brown Diamonds 3.1.2.3.1.2. TypeType IIII BrownBrown DiamondsDiamonds Ten type II diamonds—nine type IIa and one type IIb—were part of this study. They ranged in TenTen typetype IIII diamonds—ninediamonds—nine typetype IIaIIa andand oneone typetype IIb—wereIIb—were partpart ofof thisthis study.study. TheyThey rangedranged inin color from brown to pinkish brown to yellow brown, and they showed a wide range of fluorescent colorcolor fromfrom brownbrown toto pinkishpinkish brownbrown toto yellowyellow brown,brown, andand theythey showedshowed aa widewide rangerange ofof fluorescentfluorescent responses, hence covered what one can typically find in type II diamonds. The only sample missing in responses,responses, hencehence coveredcovered whatwhat oneone cancan typicallytypically findfind inin typetype IIII diamonds.diamonds. TheThe onlyonly samplesample missingmissing the selection was a stone with distinct NV center fluorescence, but samples analyzed in the past have inin thethe selectionselection waswas aa stonestone withwith distinctdistinct NVNV centcenterer fluorescence,fluorescence, butbut samplessamples analyzedanalyzed inin thethe pastpast been used to fill in the data for such diamonds. havehave beenbeen usedused toto fillfill inin thethe datadata forfor suchsuch diamonds.diamonds. Microscopic Examination MicroscopicMicroscopic ExaminationExamination Just like type I DR brown diamonds, the type II counterparts exhibited brown-colored lamellae Just like type I DR brown diamonds, the type II counterparts exhibited brown-colored lamellae alongJust the like (111) type directions I DR brown when diamonds, they were the viewed type inII immersion,counterparts and exhibited extinction brown-colored (= double refraction) lamellae along the (111) directions when they were viewed in immersion, and extinction (= double refraction) along the the (111) same directions directions when when they they were were viewed viewed in immersion, in immersion and extinction and crossed (= double polarizing refraction) filters. along the same directions when they were viewed in immersion and crossed polarizing filters. These Thesealong the lamellar same directions patterns were when identical they were in viewed the tested in immersion type IIa and and IIb crossed diamonds. polarizing Together filters. with These the lamellar patterns were identical in the tested type IIa and IIb diamonds. Together with the extinction, extinction,lamellar patterns strain-related were identical interference in the colors tested were type apparent IIa and IIb in somediamonds. of the Together tested diamonds, with the butextinction, far less strain-related interference colors were apparent in some of the tested diamonds, but far less than thanstrain-related what could interference be seen in colors many were type Iapparent DR brown in diamonds.some of the The tested extinction diamonds, patterns but infar the less type than II what could be seen in many type I DR brown diamonds. The extinction patterns in the type II diamondswhat could were be generallyseen in many finer andtype more I DR regularly brown diamonds. intersecting The than extinction the often morepatterns chaotic in the and type coarse II diamonds were generally finer and more regularly intersecting than the often more chaotic and patternsdiamonds in were type Igenerally diamonds, finer and and represented more regularly what is typicallyintersecting referred than to the as aoften “tatami more pattern” chaotic in and the coarse patterns in type I diamonds, and represented what is typically referred to as a “tatami pattern” tradecoarse (Figures patterns 24 in and type 25 I ).diamonds, and represented what is typically referred to as a “tatami pattern” inin thethe tradetrade (Figures(Figures 2424 andand 25).25).

FigureFigure 24.24. AA brownbrown typetype IIaIIa diamonddiamond (sampl (sample(samplee TH TH 2.612) 2.612) in in immersionimmersion (left) (left) showing showing the the characteristic characteristic plasticplastic deformation-relateddeformation-relateddeformation-related brown brownbrown “graining” “graining”“graining” along alonalongg (111), (111),(111), and andand the thethe corresponding correspondingcorresponding extinction extinctionextinction patterns patternspatterns as shownasas shownshown under underunder crossed crossedcrossed polarizing polarizingpolarizing ﬁlters filtersfilters (right). (right).(right).

Minerals 2020, 10, 903 29 of 38 Minerals 2020, 10, x FOR PEER REVIEW 28 of 37

Figure 25. AA brown brown type type IIb IIb diamond diamond (sample (sample TH TH 2.610) 2.610) in in immersion immersion (left) (left) showing showing the characteristic characteristic plastic deformation-relateddeformation-related brownbrown “graining” “graining” along along (111), (111), and and the the corresponding corresponding extinction extinction patterns patterns as shownas shown under under crossed crossed polarizing polarizing ﬁlters filters (right), (right), equal equal to whatto what is seen is seen in typein type IIa IIa diamonds. diamonds.

Infrared and Near-Infrared SpectroscopySpectroscopy As perper definition,definition, none none of of the the type type IIa diamondsIIa diamon showedds showed any measurableany measurable traces traces of nitrogen-related of nitrogen- absorptionsrelated absorptions in the IR in spectra; the IR hydrogen spectra; washydrogen also undetectable. was also undetectable. The type IIb diamondThe type showed—alsoIIb diamond pershowed—also definition—weak per definition—wea boron-related absorption,k boron-related the strongest absorption, of which the strongest being a relatively of which sharp being peak a 1 atrelatively 2800 cm sharp− . The peak spectra at 2800 of cm type−1. The IIa DRspectra brown of type diamonds IIa DR exhibited brown diamonds very weak exhibited AC absorption very weak at 1 1 4063AC absorption cm− and aat broader4063 cm band−1 and at a 4200broader cm− band. Verifying at 4200 thecm− laboratory’s1. Verifying the database, laboratory’s it was database, found that it virtuallywas found all that type virtually IIa brown all diamonds type IIa brown ever analyzed diamonds at theever laboratory analyzed did at the exhibit laboratory a weak did to very exhibit weak a 1 1 4063weak cm to− veryAC. weak With the4063 4063 cm cm−1 AC.− AC With being the the 4063 least cm stable−1 AC in being type Iathe and least Ib diamonds,stable in type it was Ia ofand great Ib interestdiamonds, to seeit was how of thisgreat AC interest behaved to see in typehow IIthis diamonds, AC behaved and in therefore type II diamonds, heat treatment and therefore at 1500 ◦C heat for twotreatment hours at was 1500 also °C performed for two hours on type was IIa also DR brownperformed diamonds. on type The IIa experiment DR brown revealed diamonds. that The the ACexperiment in the tested revealed type that IIa DRthe brownAC in the diamonds tested type was notIIa DR modified brown by diamonds annealing was at thesenot modified conditions; by 1 theannealing 4063 cm at− theseAC remained conditions; unaff theected. 4063 Comparing cm−1 AC theremained behavior unaffected. between type Comparing I and type IIathe diamonds, behavior abetween possible type explanation I and type for IIa this diamonds, differing annealing a possible reaction explanation could befor thethis absence differing of nitrogen;annealing this reaction could 1 1 1 implicatecould be the that absence in order of tonitrogen; form the this 4167 could cm implicate− and/or that the 4111in order cm− to ACsform out the of4167 the cm 4063−1 and/or cm− ACthe 1 nitrogen4111 cm−1 needs ACs out to beof the available. 4063 cm From−1 ACthis nitrogen it could needs be concludedto be available. that theFrom 4063 this cm it −couldAC be defect concluded might possiblythat the 4063 not containcm−1 AC nitrogen. defect might possibly not contain nitrogen. In most low temperature NIR spectra of the analyzed type II DR brown diamonds, a sharp ZPL 1 at 7878 cm−1 (=(= 1269.21269.2 nm) nm) was was detected; detected; heating heating at at 1500 1500 °C◦C reduced reduced the the intensity of this absorption 1 by 80%. In the type IIb diamond, a similar ZPL was foundfound slightlyslightly shifted,shifted, atat 78687868 cmcm−−1 (1271.0(1271.0 nm), nm), 1 at LNT. In In the the NIR NIR spectrum spectrum of of the the pink pink brown brown sample sample (TH (TH 2.584), 2.584), a ZPL a ZPL at 4847 at 4847 cm− cm1 and− oneand at one 7497 at 1 1 7497cm−1 were cm− detected;were detected; the peak the at peak 4847 at cm 4847−1 was cm− foundwas foundto anneal to anneal out after out 2 afterh at 1500 2 h at °C. 1500 ◦C.

UV-Vis-NIR SpectroscopySpectroscopy As with type I brown diamonds, the color of ty typepe II DR brown diamonds is the result of the relatively ﬂatflat and steadily increasingincreasing continuumcontinuum fromfrom thethe NIRNIR toto UV.UV. With nitrogen-relatednitrogen-related absorptions, absorptions, such such as N3 as and N3 H3, and typically H3, typically absent, theabsent, only featuresthe only overlaying features theoverlaying continuum the continuum were a broad were band a broad at 675 band nm, at and 675 for nm, the and very for unusual the very samples unusual with samples the 597.9 with nmthe PL597.9 (see nm PL PL section (see PL for section details), for a details), broad band a broad at 550 band nm andat 550 a doublenm and band a double at 385 band/396 nmat 385/396 was found nm (Figurewas found 26); (Figure the 550 26); nm the band 550 nm caused band the caused pinkish the pinkish color seen color in seen the diamondin the diamond shown shown in image in image d in Figured in Figure 26. This26. This 550 550 nm–385 nm–385/396/396 nm nm spectrum spectrum is identical is identical to what to what is seen is seen for for any any type type IIa IIa diamond diamond of 0 pinkishof pinkish brown brown color, color, with with exception exception of theof the rare rare cases cases when when the the NV NVcenter0 center causes causes a pinkish a pinkish color color in suchin such diamonds diamonds [25 [25,26].,26].

Minerals 2020, 10, 903 30 of 38 Minerals 2020, 10, x FOR PEER REVIEW 29 of 37

Figure 26. The didifferentﬀerent UV-Vis-NIRUV-Vis-NIR spectra spectra of of brown brown type type II diamonds,II diamonds, recorded recorded at 77 at K. 77 As K. with As with type typeI brown I brown diamonds, diamonds, the color the ofcolor such of stones such isstones the result is the of result the relatively of the relatively ﬂat and steadily flat and increasing steadily increasingcontinuum continuum from the NIR from to UV. theWith NIR nitrogen-relatedto UV. With nitrogen-related absorptions, such absorptions, as N3 and such H3, typicallyas N3 and absent, H3, typicallythe only featuresabsent, the overlaying only features the continuum overlaying arethe acontinuum broad band are at a 675broad nm, band and at for 675 the nm, very and unusual for the verysamples unusual with thesamples 597.9 with nm PL,the a597.9 broad nm band PL, ata 550broad nm band and aat double 550 nm band and ata double 385/396 band nm can at be385/396 seen, whichnm can is be basically seen, which a regular is basically UV-Vis-NIR a regular spectrum UV for-Vis-NIR any pink spectrum brown typefor any IIa diamond.pink brown The type spectra IIa werediamond. shifted The vertically spectra were for clarity. shifted vertically for clarity.

Photoluminescence Spectroscopy The PLPL spectraspectra of of type type IIa IIa diamonds diamonds were were found found to be veryto be variable, very variable, since many since type many IIa diamonds type IIa diamondsare simply are type simply I diamonds type I diamonds with a very with low a nitrogenvery lowcontent. nitrogen Hence, content. many Hence, defects many found defects in found type I indiamonds type I diamonds are also found are also in typefound IIa in diamonds. type IIa di Inamonds. consequence, In consequence, the PL spectra the PL shown spectra in shown Figure 17in Figureabove of17 a above low nitrogen of a low type nitrogen Ia diamond type Ia is adiamond typical example is a typical of spectra example as theyof spectra also occur as they for manyalso occur type forIIa diamonds.many type BesidesIIa diamonds. 406.1 nm, Besides N3, H3,406.1 490.8 nm, nm, N3, 535.9H3, 490.8 nm, H4,nm, 576.0535.9 nm,nm, andH4, 612.5576.0 nm,nm, whichand 612.5 can nm,typically which be can found typically in the PLbe spectrafound in of the both PL type spectra Ia and of typeboth IIa type diamonds, Ia and type some IIa features diamonds, are mainly some featuresfound in are the mainly spectra found of low in the nitrogen spectra type of low Ia and nitrogen “nitrogen-free” type Ia and type“nitrogen-free” II diamonds, type including II diamonds, type includingIIb. These type include IIb. 417.2These nm, include 431.1 417.2 nm, 3Hnm, (503.5 431.1 nmnm, ZPL),3H (503.5 558.3 nm nm, ZPL), 566.0 558.3 nm, nm, 566.7 566.0 nm, nm, 567.6 566.7 nm, nm,578.9 567.6 nm, 649.5nm, 578.9 nm, distinctnm, 649.5 GR1 nm, (741.2 distinct/744.6 GR1 nm ZPL),(741.2/744.6 802.2, andnm 814.2ZPL), nm.802.2, Some and of 814.2 these nm. features Some are of theseseen infeatures Figure are27, seen the PL in spectraFigure 27, of thethe onlyPL spectr typea IIb of DR the brownonly type diamond IIb DR included brown diamond in this study; included as a ingeneral this study; rule, aas non-type-sectored a general rule, a non-type-sectored type IIb diamond willtype notIIb exhibitdiamond any will nitrogen-related not exhibit any PL nitrogen- centers, relatedsuch as PL it is centers, the case such for thisas it speciﬁcis the case stone. for this Interestingly, specific stone. the few Interestingly, truly brown the type few IIb truly diamonds brown type that weIIb havediamonds seen sothat far we always haveexhibited seen so far very always similar exhibited spectral very data likesimilar the samplespectral included data like in the this sample study, withincluded the 417.2in this nm study, defect with being the the 417.2 consistent nm defect one being present the in consistent all the spectra. one present Only samples in all the appearing spectra. “olive”Only samples or blue withappearing a brownish “olive” tinge or blue that wewith have a br testedownish in thetinge past that exhibited we have the tested PL centers in the typical past exhibitedfor gray to the blue PL type centers IIb diamonds,typical for suchgray asto theblue 479.9 type nm, IIb 517.6diamonds, nm, 648.1 such nm, as the and 479.9 776.4 nm, nm 517.6 defects. nm, 648.1 nm, and 776.4 nm defects. Some rare type IIa DR brown diamonds can exhibit PL spectra characterized by mainly NV centers and other radiation induced defects; such stones have been tested prior to this study, but no stones are directly included here; the PL features typically seen include 388.8 nm, 470.0 (TR12), H3, 550.8 nm, NV0, NV−, and GR1. Often, such stones are of mixed pink and brown color.

Minerals 2020, 10, 903 31 of 38 Minerals 2020, 10, x FOR PEER REVIEW 30 of 37

310

FigureFigure 27. TheThe photoluminescence photoluminescence spectra spectra of of a a rare rare type type IIb IIb brown brown diamond, recorded recorded using using four differentdiﬀerent lasers. With With many many of of the the features features being being characteristic characteristic of of low low nitrogen nitrogen Ia Ia or or type type II II diamonds, diamonds, thethe asymmetric asymmetric and somewhat largelarge bandband atat 417.2417.2 nm nm is is only only rarely rarely found found in in type type IIa IIa diamonds diamonds but but is isvery very common common in in type type IIb IIb diamonds. diamonds. The The spectra spectra were were shifted shifted vertically vertically for for clarity. clarity.

InSome very rare large type parcels IIa DR brownof small diamonds (<0.02 ct) can pinkish exhibit brown PL spectra diamonds characterized we have by mainlyencountered NV centers four highlyand other unusual radiation nickel-rich induced type defects; IIa DR such pinkish stones brown have been diamonds tested prior with to distinct this study, orangey but no red stones PL, not are relateddirectly to included the NV here;centers; the the PL series features of PL typically spectra seen characteristic include 388.8 for nm,these 470.0 diamonds, (TR12), recorded H3, 550.8 from nm, 0 sampleNV , NV TH−, 2.584, and GR1. are shown Often, suchin Figure stones 28. are Besides of mixed the 417 pink nm and PL brown peak described color. above and a distinct doubletIn veryat 882.9/884.6 large parcels nm—a of small well-known (<0.02 ct) nickel-related pinkish brown defect diamonds assigned we to have Nii+ encountered [27]—the spectra four highly were foundunusual to nickel-richbe very unusual type IIa with DR all pinkish other brownPL featur diamondses unknown with distinctin our PL orangey database. red PL,This not very related rare orangeyto the NV red centers; PL was the found series to of originate PL spectra from characteristic three very sharp for these and diamonds, intense ZPL’s recorded at 559.7 from nm, sample 584.5 nm,TH 2.584,and 597.9 are shown nm, with in Figure their 28associated. Besides vibronic the 417 nm sidebands PL peak that described range aboveall the and way a distinctto 850 nm doublet and + whichat 882.9 are/884.6 centered nm—a at well-known approximately nickel-related 650 nm. After defect some assigned research to Ni intoi [ 27these]—the spectral spectra features, were found it was to discoveredbe very unusual that withthese all othertypes PLof featuresspectra unknown are reminiscent in our PL of database. what has This verybeen raredescribed orangey in red the PL cathodoluminescencewas found to originate spectra from three of natural very sharp type andIIa di intenseamonds ZPL’s that athave 559.7 been nm, implanted 584.5 nm, by and nickel 597.9 and nm, oxygenwith their ions associated and annealed vibronic to 1650 sidebands °C [28]. thatNonetheles range alls, it the was way confirmed to 850 nm by andthe authors which are of centered[28] that theat approximately spectra of their 650research nm. Afterwere not some identical research to th intoe ones these included spectral here features, and the it waslack of discovered features such that asthese H3 typesand/or of spectraNV centers are reminiscent are highly of indicative what has beenthat no described artificial in theirradiation cathodoluminescence and annealing spectra were involved;of natural while type IIaannealing diamonds at 1650 that °C have would been anneal implanted out all by irradiation-related nickel and oxygen features, ions and H3 annealed and/or NV to centers1650 ◦C[ would28]. Nonetheless, clearly form during it was conﬁrmedsuch a treatment by the and authors survive of [28 the] thatannealing. the spectra of their research wereWe not have identical heated to thethe onessample included included here here and at the 1500 lack °C of for features two hours such to as see H3 whether and/or NV the centerstreatment are wouldhighly indicativemodify the that PL no of artiﬁcialthe diamond, irradiation but it and had annealing no effect were whatsoever. involved; The while UV-Vis-NIR annealing atdata 1650 also◦C suggestswould anneal that the out diamonds all irradiation-related might be naturally features, colo H3red, and since/or NVthe continuum centers would absorption clearly formwith the during 550 nmsuch band a treatment combined and with survive the double the annealing. absorption at 385/396 nm is in no way different to a regular type IIa pink brown diamond. The results from the NIR spectroscopy performed for the stone that was heated at 1500 °C shows at least that this sample has for sure not been heated above 1400 °C, since an NIR peak at 4847 cm−1 was found to anneal out after 2 h at 1500 °C. In conclusion, these unusual diamonds remain puzzling, but combining all data it is highly likely that these diamonds are naturally colored and untreated.

Minerals 2020, 10, 903 32 of 38 Minerals 2020, 10, x FOR PEER REVIEW 31 of 37

340

Figure 28. The photoluminescence spectra spectra of of a highly unusual pink brown type IIa diamond with orangey redred luminescenceluminescence not not originating originating from from the the NV NV0 center,0 center, but but from from the the 559.7, 559.7, 584.5, 584.5, and and 597.9 597.9 nm defects,nm defects, excited excited using using three three diﬀ differenterent lasers. lasers. These These are are nickel-containing nickel-containing natural natural typetype IIa diamonds, as is apparent by the presence of the 882.9/884.6 nm Nii++ doublet; the 584.5 and 597.9 nm defect(s) can as is apparent by the presence of the 882.9/884.6 nm Nii doublet; the 584.5 and 597.9 nm defect(s) can possibly be attributed to oxygen or nickel defects. The spectra were shifted vertically for clarity.

DFI FluorescenceWe have heated Imaging the sample and Spectroscopy included here at 1500 ◦C for two hours to see whether the treatment would modify the PL of the diamond, but it had no effect whatsoever. The UV-Vis-NIR data also As mentioned above, the PL of type II diamonds can be very variable because of the fact that suggests that the diamonds might be naturally colored, since the continuum absorption with the type II does not mean that a stone is strictly nitrogen-free, but only that nitrogen cannot be detected 550 nm band combined with the double absorption at 385/396 nm is in no way different to a regular by infrared spectroscopy. As a consequence, the PL imaging is rather diverse, and a range of different type IIa pink brown diamond. The results from the NIR spectroscopy performed for the stone that was colors and color distributions can be observed, depending on the defects present. As a general rule it heated at 1500 C shows at least that this sample has for sure not been heated above 1400 C, since can be stated that◦ the fluorescence intensity in a type II diamond is lowest when the density◦ of PL an NIR peak at 4847 cm 1 was found to anneal out after 2 h at 1500 C. In conclusion, these unusual active defects is lowest −and/or when the brown color of the diamonds◦ is the darkest. In diamonds diamonds remain puzzling, but combining all data it is highly likely that these diamonds are naturally with excessive PL active defects the fluorescence intensity decreases, because of the resulting colored and untreated. imperfection of the diamond lattice. Equally, when a diamond is very rich in DR defects, such as DFIvacancy Fluorescence clusters, Imagingthe defective and Spectroscopy lattice causes a suppression of fluorescence. A highly defective diamond lattice gives rise to many alternative pathways for de-excitation of the UV-excited electrons, and notAs mentionedall of these above, result the in PLfluorescence. of type II diamonds Energy transfers can be very that variable lead to becausethis so-called of the factfluorescence that type IIquenching does not are mean well that known a stone in isdiamonds strictly nitrogen-free, very rich in butaggregated only that nitrogen, nitrogen which cannot fluoresce be detected much by infraredweaker than spectroscopy. stones much As alower consequence, in nitrogen, the PLeven imaging though is the rather stones diverse, with and much a range higher of dinitrogenfferent colorscontent and are colormuch distributions richer in N3 cancenters be observed, than the lowe dependingr nitrogen on thesamples. defects Therefore, present. Ashigh a generalnitrogen rule type it canIaAB be yellow stated “cape” that the diamonds fluorescence barely intensity show inany a typeN3-related II diamond blue fluorescence is lowest when while the relatively density of low PL activenitrogen defects type isIaAB lowest near and colorless/or when “cape” the brown diamonds color of typically the diamonds fluoresce is the much darkest. stronger. In diamonds This energy with excessivetransfer occurs PL active from defects the N3 the center fluorescence to the A a intensityggregate decreases, [29], possibly because also ofthe the B aggregate resulting imperfection[30]. of theAs diamond a consequence, lattice. Equally,the most when intense a diamond fluorescence is very was rich found in DR in defects, lightlysuch colored as vacancy “near-type clusters, IIa” thediamonds, defective stones lattice in causes which a suppressionthe IR spectrum of fluorescence. does not Adefinitely highly defective define diamondthem as IIa, lattice but gives neither rise todefinitive many alternative nitrogen-related pathways absorptions for de-excitation can be ofdefi thened, UV-excited hence stones electrons, with anda nitrogen not all ofcontent these resultof <2 inppm. fluorescence. Such stones Energy appear transfers very similar that leadto type to this Ia DR so-called brown fluorescencediamonds when quenching tested areunder well the known various in diamondsexcitations veryof the rich DFI in aggregated system, such nitrogen, as seen which in Figure fluoresce 29; much the weakershown diamond than stones exhibited much lower green in nitrogen,fluorescence even from though the H4 the and stones 490.7 with nm much defects higher and blue nitrogen fluorescence content arefrom much the 406.1 richer nm in and N3 centersthe N3 thandefects, the similar lower nitrogento many samples.type Ia DR Therefore, brown diamonds. high nitrogen type IaAB yellow “cape” diamonds barely showOrange any N3-related to pink/red blue fluorescence fluorescence does while occur relatively very rarely low nitrogen in type type IIa DR IaAB brown near colorlessdiamonds, “cape” and diamondsgenerally this typically relates fluoresce to the NV much0 center stronger. [26]. While This energysuch diamonds transfer occurshave been from tested the N3 by centerus during to the this A aggregatestudy, no such [29], possiblysample is also included the B aggregate here. The [ 30diamonds]. that are included are very unusual pinkish brown diamonds with a distinct pink to orange red PL under the various excitations used (Figure 30); while it appeared obvious that these should be luminescent from the NV0 center, the spectra of

Minerals 2020, 10, 903 33 of 38

As a consequence, the most intense fluorescence was found in lightly colored “near-type IIa” diamonds, stones in which the IR spectrum does not definitely define them as IIa, but neither definitive nitrogen-related absorptions can be defined, hence stones with a nitrogen content of <2

Mineralsppm. Such 2020, 10 stones, x FOR appear PEER REVIEW very similar to type Ia DR brown diamonds when tested under the various32 of 37 excitations of the DFI system, such as seen in Figure 29; the shown diamond exhibited green ﬂuorescence thefrom emission the H4 and identified 490.7 nm it defectsas the and559.7, blue 584.5, ﬂuorescence and 597.9 from nm thedefects 406.1 and nm andtheir the associated N3 defects, vibronic similar sidebands;to many type these Ia DRdefects brown are diamonds. possibly linked to oxygen or nickel.

Minerals 2020, 10, x FOR PEER REVIEW 32 of 37

the emission identified it as the 559.7, 584.5, and 597.9 nm defects and their associated vibronic sidebands; these defects are possibly linked to oxygen or nickel.

FigureFigure 29. A light yellowishyellowish brownbrown nearnear type type IIa IIa diamond diamond (sample (sample TH TH 2.616) 2.616) under under three three diﬀ differenterent UV UVexcitations excitations of the of the DFI DFI system. system. The The green green PL PL is caused is caused by theby the 490.7 490.7 nm nm and and H4 H4 defects, defects, the the blue blue PL PL by bythe the 406.1 406.1 nm nm and and the the N3 N3 defects. defects.

Orange to pink/red ﬂuorescence does occur very rarely in type IIa DR brown diamonds, and generally this relates to the NV0 center [26]. While such diamonds have been tested by us during this study, no such sample is included here. The diamonds that are included are very unusual

pinkish brown diamonds with a distinct pink to orange red PL under the various excitations used (FigureFigure 30); while29. A light it appeared yellowish obvious brown that near these type shouldIIa diam beond luminescent (sample TH from 2.616) the under NV0 threecenter, different the spectra of theUV emission excitations identiﬁed of the DFI it system. as the 559.7,The green 584.5, PL is and caused 597.9 by nmthe 490.7 defects nm andand H4 their defects, associated the blue vibronic PL sidebands;by the these406.1 nm defects and the are N3 possibly defects. linked to oxygen or nickel.

Figure 30. A pink brown type IIa diamond (sample TH 2.584) under three different UV excitations of the DFI system. The pink to orange red PL is caused by the possibly oxygen- or nickel-related 559.7, 584.5, and 597.9 nm defects.

3.2. Comparison of DR Brown with DR Pink to Red to Purple Diamonds As mentioned in the microscopic examination and UV-Vis-NIR spectroscopy sections above, there is an obvious link between brown DR diamonds and pink to red to purple diamonds.. With brownFigureFigure diamonds 30.30. A pink often brown exhibiting type IIa both, diamond brown (sample and THpink 2.584) color under lamellae, three didifferentandfferent often UVUV showing excitationsexcitations spectral ofof featuresthethe DFIinDFI their system.system. UV-Vis-NIR The pinkpink to spectra orangeorange that redred PL PLare isis char causedcausacteristiced byby thethe possiblypossiblyof pink oxygen-oxygen-diamonds, or nickel-relatednickel-related the obvious 559.7,question must584.5, 584.5,be what andand the 597.9597.9 differences nmnm defects.defects. between the two clearly related color groups are. For this reason, a series of3.2. pink Comparison diamonds of DRwere Brown specifically with DR analyzed Pink to Red for toth Purpleis study Diamonds and the data of large numbers of pink to purple3.2. Comparison to red diamonds of DR Brown analyzed with DR in Pinkthe pastto Red at to our Purple laboratory Diamonds was reviewed and included. Pink As mentioned in the microscopic examination and UV-Vis-NIR spectroscopy sections above, diamondsAs mentioned do not exhibit in the the microscopic characteristic examination UV-Vis-NIR and continuumUV-Vis-NIR absorption spectrosco responsiblepy sections for above, the there is an obvious link between brown DR diamonds and pink to red to purple diamonds. With brown brownthere is color an obvious but instead link they between exhibit brown a broad DR absorption diamonds band and pinkcentered to red at 550to purpleto 560 nm,diamonds. often with With a diamonds often exhibiting both, brown and pink color lamellae, and often showing spectral features bandbrown at diamonds390 nm, which often consists exhibiting typically both, ofbrown two components and pink color at 385 lamellae, and 396 and nm. often Their showing infrared spectralspectra in their UV-Vis-NIR spectra that are characteristic of pink diamonds,−1 the obvious−1 question−1 must be lackfeatures the distinctin their ACUV-Vis-NIR absorptions spectra but thatvery are weak char ACsacteristic at 4063 of cmpink, diamonds,4167 cm , theor 4200obvious cm question can be what the differences between the two clearly related color groups are. For this reason, a series of pink detectedmust be whatin some the differencesof the type between Ia stones. the Besides two clearly type related IIa brown color diamonds, groups are. type For thisIa pink reason, to purplea series diamonds were specifically analyzed for this study and the data of large numbers of pink to purple−1 diamondsof pink diamonds are the only were diamonds specifically for analyzed which we for ha thveis occasionally study and the detected data of an large isolated numbers 4063 ofcm pink AC, to to red diamonds analyzed in the past at our laboratory was reviewed and included. Pink diamonds evenpurple if only to red weak. diamonds In contrast analyzed to brown in thediamonds, past at noneour laboratory of the pink was to purple reviewed diamonds and included. with the 4063Pink do− not1 exhibit the characteristic UV-Vis-NIR continuum absorption responsible for the brown color cmdiamonds AC showed do not any exhibit H1b and/orthe characteristic H1c center UV-Vabsorption.is-NIR continuum absorption responsible for the brownTo colorsee how but theinstead weak they ACs exhibit in pink a broad to red absorption to purple diamondsband centered reacted at 550 to toheat, 560 five nm, type often Ia with pink a diamondsband at 390 were nm, heatedwhich consiststo 1500 °Ctypically for two of hours; two components interestingly, at 385 none and of 396 the nm. weak Their ACs infrared changed spectra with −1 thelack annealing, the distinct including AC absorptions the 4063 cm but ACvery that weak was ACs shown at to4063 be socm noteworthy−1, 4167 cm− unstable1, or 4200 in cm DR−1 browncan be diamondsdetected in from some temperatures of the type as Ia low stones. as 1150 Besides °C. On type the noteIIa brown of heat diamonds, treatment, typethe behavior Ia pink ofto certainpurple typediamonds IIa DR are brown the only diamonds diamonds to HPHTfor which treatment we have sh occasionallyould be noted, detected since anstones isolated of pinkish 4063 cm brown−1 AC, coloreven mayif only turn weak. pink In whencontrast HPHT to brown treated diamonds, at lower none temperature of the pink of to <2000 purple °C diamonds [11,31], and with then the turn4063 colorlesscm−1 AC orshowed yellowish any H1bwhen and/or HPHT H1c treated center in absorption.a second step at >2000 °C [31].

To see how the weak ACs in pink to red to purple diamonds reacted to heat, five type Ia pink diamonds were heated to 1500 °C for two hours; interestingly, none of the weak ACs changed with the annealing, including the 4063 cm−1 AC that was shown to be so noteworthy unstable in DR brown diamonds from temperatures as low as 1150 °C. On the note of heat treatment, the behavior of certain type IIa DR brown diamonds to HPHT treatment should be noted, since stones of pinkish brown color may turn pink when HPHT treated at lower temperature of <2000 °C [11,31], and then turn colorless or yellowish when HPHT treated in a second step at >2000 °C [31].

Minerals 2020, 10, 903 34 of 38 but instead they exhibit a broad absorption band centered at 550 to 560 nm, often with a band at 390 nm, which consists typically of two components at 385 and 396 nm. Their infrared spectra lack the 1 1 1 distinct AC absorptions but very weak ACs at 4063 cm− , 4167 cm− , or 4200 cm− can be detected in some of the type Ia stones. Besides type IIa brown diamonds, type Ia pink to purple diamonds are the 1 only diamonds for which we have occasionally detected an isolated 4063 cm− AC, even if only weak. 1 In contrast to brown diamonds, none of the pink to purple diamonds with the 4063 cm− AC showed any H1b and/or H1c center absorption. To see how the weak ACs in pink to red to purple diamonds reacted to heat, five type Ia pink diamonds were heated to 1500 ◦C for two hours; interestingly, none of the weak ACs changed with the 1 annealing, including the 4063 cm− AC that was shown to be so noteworthy unstable in DR brown diamonds from temperatures as low as 1150 ◦C. On the note of heat treatment, the behavior of certain type IIa DR brown diamonds to HPHT treatment should be noted, since stones of pinkish brown color may turn pink when HPHT treated at lower temperature of <2000 ◦C[11,31], and then turn colorless or yellowish when HPHT treated in a second step at >2000 ◦C[31]. Clearly, the nature of the defect responsible for the pink color—even though certainly similar to the vacancy cluster—cannot be the same as the one in DR brown diamonds. Pink diamonds lacking any brown graining do not show the complex DR PL spectra that are so characteristic for brown diamonds. With the exception of the H3 and H4 center, deformation-related defects appear absent in the PL spectra, even though the color origin is clearly deformation-related. The question thus remains, what defects can be created by post-growth plastic deformation in a way that both pink and brown color lamellae can be present at the same time? There is no significant difference other than this lack of IR and PL active deformation-related defects, both DR brown and pink diamonds can be type IIa, both type Ia DR brown and pink diamonds are relatively low in nitrogen, low to very low in hydrogen and can show any kind of nitrogen aggregation state, with often B>A in pink diamonds and A>>B in more purple diamonds. It seems obvious that both brown and pink to red to purple diamonds have the same formation conditions; the difference between the color groups is thus not their growth condition, but the post-growth conditions. As post-growth plastic deformation is the process responsible for these colors, the conditions of plastic deformation must be significantly different to cause one or the other color. A multi-step deformation process is the only way to explain both pink and brown color lamellae in the same stone. From the annealing behavior of the amber center, which has shown that 1 1 the 4063 cm− AC is the least stable AC and anneals out at 1500 ◦C to form both the 4165 cm− AC 1 or the 4111 cm− AC (if C centers are available) and from the creation of the different ACs via heavy neutron irradiation and annealing, it seems logical to conclude that the pink to red to purple color is likely caused by plastic deformation at lower temperatures while the brown color is likely caused by plastic deformation at higher temperatures. Two or more different plastic deformation events at significantly different temperatures would thus be a logical way of explaining the occurrence of pink and brown color lamellae in the same diamond.

3.3. Treated Brown Diamonds with Originally DR Brown Color A series of type Ia brown diamonds was included in this work, which had been electron irradiated and annealed. While some of the stones showed an obviously modified color—being yellow to greenish yellow after irradiation and annealing—some stones remained essentially brown during the entire process. The reason for this was generally the starting color of the diamond: when the color was comparatively light, then the resulting color was as desired (greenish blue after irradiation, yellow to greenish yellow after annealing) (Figure 31), but when the color was somewhat darker, the resulting colors were brownish or even dominantly brown (Figure 32). The explanation for this is the combination of the original color with the diamond type and the nitrogen content, plus the energy of the electrons and the dose used: it is difficult to distinctly modify an overly dark brown color with the defects induced by irradiation and annealing. The treatment does not change the originally occurring UV-Vis-NIR absorption continuum caused by the vacancy clusters, and GR1, H3, or H4 are Minerals 2020, 10, 903 35 of 38 generally not strong enough to mask the brown color. The nitrogen type and concentration are of importance since it is much easier to create significant H3 absorption compared to H4; in consequence it is far more efficient to create yellow color in diamonds with significant concentrations of A aggregates than in pure of nearly pure type IaB diamonds. The two examples shown in Figures 31 and 32 were both diamonds with mainly B aggregates; the main differences between the two was the depth of the brown color, with sample TH 2.211 being significantly lighter that sample TH 2.212, and that the lighter stone had three times higher nitrogen concentrations than the darker diamond. As a consequence, the treatment worked nicely for the lighter diamond, while the darker one remained essentially brown at each step of the treatment. Minerals 2020,, 10,, xx FORFOR PEERPEER REVIEWREVIEW 34 of 37

Figure 31.31. The evolution of the color and the UV-Vis-NIR spectraspectra ofof aa lightlight brownbrown diamonddiamond (sample(sample

THTH 2.211) after after 2 2 h h of of 2 2MeV MeV electron electron irradiation irradiation and and stepwise stepwise annealing annealing from from 300 300°C to◦C 1310 to 1310 °C. The◦C. Thestone stone showed showed the thedesired desired color color changes changes from from brown brown via via greenish greenish blue blue to to greenish greenish yellow, yellow, by formationformation ofof GR1GR1 fromfrom irradiationirradiationirradiation andanand H4H4 plusplus H3H3 viavia thethe annealing.annealing.

Figure 32. TheThe evolution evolution of of the the color color and and the the UV-Vis-NIR UV-Vis-NIR spectra spectra of of a a darkerdarker brownbrown diamond diamond (sample (sample TH 2.212) after 2 h of 10 MeV electron irradiation and stepwise annealing from 300 °C to 1310 °C. The TH 2.212) after 2 h of 10 MeV electron irradiation and stepwise annealing from 300 ◦C to 1310 ◦C. Thestone stone showed showed some some color color changes changes but remained but remained essentially essentially brown, brown, because because neither neither GR1 GR1 nor norH4 or H4 H3 or H3had had a strong a strong enough enough influence inﬂuence on th one thedistinct distinct continuum continuum absorption. absorption.

3.4. The Classification of Natural Brown Diamonds with Deformation-Related Color InIn contrastcontrast toto thethe originaloriginal classificationclassification proposedproposed inin 20032003 [6],[6], thethe newnew classificationclassification putsputs thethe group of brown diamonds into two main groups,, deformation-relateddeformation-related (DR)(DR) brownbrown diamondsdiamonds andand non-deformation-related (NDR) brown diamonds. In this firstfirst partpart ofof thisthis studystudy thethe classificationclassification ofof thethe DRDR brownbrown diamondsdiamonds isis presented,presented, inin thethe sesecond part the classification of the NDR brown diamonds will be presented. The DR brown diamonds are subdivided into type I and type II, and fromfrom herehere intointo typetype Ia,Ia, typetype Ib,Ib, typetype IIa,IIa, andand tytype IIb. The type I diamonds areare furtherfurther divideddivided byby theirtheir typicaltypical amberamber centers,centers, whichwhich leadsleads toto aa totaltotal ofof sixsix differentdifferent classesclasses ofof DRDR brownbrown diamondsdiamonds (see(see Table 4 for details and properties).

Minerals 2020, 10, 903 36 of 38

Other examples of diamonds that turn brown by treatment include DR type Ib deep yellow-orange to olive diamonds; these may appear mainly brown after simple irradiation, or turn orange brown after annealing. Here the brown hue is caused by combination of the strong continuum absorption from the C center and deformation-related vacancy clusters with the absorptions induced by irradiation and annealing (ND1, GR1, and NV−)[19].

3.4. The Classification of Natural Brown Diamonds with Deformation-Related Color In contrast to the original classification proposed in 2003 [6], the new classification puts the group of brown diamonds into two main groups, deformation-related (DR) brown diamonds and non-deformation-related (NDR) brown diamonds. In this first part of this study the classification of the DR brown diamonds is presented, in the second part the classification of the NDR brown diamonds will be presented. The DR brown diamonds are subdivided into type I and type II, and from here into type Ia, type Ib, type IIa, and type IIb. The type I diamonds are further divided by their typical amber centers, which leads to a total of six different classes of DR brown diamonds (see Table4 for details and properties).

Table 4. The newly elaborated classiﬁcation of natural, untreated DR brown diamonds.

Deformation-Related (DR) Brown Diamonds Main type Type I Type II Sub type Type Ia Type Ib Type IIa Type IIb Main AC 4167 cm 1 4167/4063 cm 1 4111/4063/3462 cm 1 None or − − 4111 cm 1 AC − None (RT) AC ACs − ACs very weak Other Higher C center May exhibit H1b No Y center characteristic None content, Y center, None None and H1c present IR features may show H1b 0 Characteristic 0 565.7, H3, NV , 617.6/626.3 nm, 612.4 nm H3, NV , NV− Various 417.2 PL features NV− Absorption Steeper absorption Characteristic Absorption continuum from NIR continuum from continuum from Absorption continuum from UV-Vis-NIR to the UV, 560 nm band NIR to the UV, NIR to the UV, weak NIR to the UV features NV− center NV− center 4167 AC DR 4167/4063 DAC 4111 AC IIa DR IIb DR 4111/4063/3462 TAC Class name brown DR brown Ib DR brown brown brown Ib brown diamond diamond diamond diamond diamond diamond

4. Conclusions In the first part of this study, we have given a full characterization of DR brown diamonds of all types and presented a re-worked classification of this group of diamonds. The extensive amount of data obtained from the diamonds studied and from diamonds studied for other projects, such as our diamond irradiation project, has led to significant advances in the comprehension of some of the characteristics of brown diamonds and on the link between pink to red to purple and brown diamonds. Particularly important is the data on the amber center and the naturally occurring H1b and H1c defects found in untreated DR brown type I diamonds as well as the detailed PL characterization of DR brown diamonds. The re-worked classification still divides diamonds into such that exhibit IR spectra with amber centers, and such that do not show any amber centers, but it puts the entire group into two major classes, one being with a deformation-related (DR) brown color and one being with a non-deformation-related (NDR) brown color. The DR brown diamonds were finally divided into six different classes, four with ACs and two without ACs. The detailed study of the ACs and the natural H1b and H1c centers by a range of methods, including low temperature NIR spectroscopy and photoexcitation, and the analysis of the behavior of these features upon annealing, electron irradiation, and finally electron irradiation and annealing, have given a much deeper insight into these defects. The various ACs were found to be closely related Minerals 2020, 10, 903 37 of 38

1 defects, most likely involving multiple vacancies and nitrogen, with the 4063 cm− AC being the 1 1 precursor of the 4167 cm− group of ACs and very likely also the 4111 cm− group of ACs; there are 1 indications that the 4063 cm− AC does not contain nitrogen but this must be investigated in more 1 detail. The defect structure of the 4167 cm− AC proposed in an earlier study [8] seems an unlikely candidate to explain this complex group of defects, and defects involving more complex vacancy accumulations seem more likely. It is suggested that the naturally occurring H1b and H1c centers 1 1 in brown diamonds are side products of the transformation of the 4063 cm− AC into the 4167 cm− 1 and/or 4111 cm− AC hence have a clear link to deformation-induced multi-vacancy defects. The link between DR pink to red to purple and brown diamonds was elaborated and a mechanism involving post-growth plastic deformation at different temperatures was suggested that could explain the occurrence of the different colors; in this mechanism, it is suggested that the brown color occurs when the plastic deformation occurred at a higher temperature while the pink to red to purple color occur when the plastic deformation occurred at a lower temperature. This is consistent with the conclusions drawn in earlier publications [32]. Such a mechanism would be perfectly suitable to explain the presence of both pink and brown color lamellae in one and the same diamond, by assuming that the post-growth plastic deformation process did not only occur in one single event, but in two or multiple events at significantly different temperatures.

Author Contributions: conceptualization, T.H. and G.P.; methodology, T.H.; formal analysis, T.H., G.P. and F.N.; investigation, T.H., F.N. and G.P.; writing—original draft preparation, T.H.; writing—review and editing, T.H., F.N. and G.P.; visualization, T.H., F.N. and G.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors are grateful to Andrei Katrusha for fruitful discussions. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Hainschwang, T.; Notari, F.; Pamies, G. A defect study and classification of brown diamonds with non-deformation-related color. Minerals 2020. under process. 2. Fujita, N.; Jones, R.; Öberg, S.; Briddon, P.R. Large spherical vacancy clusters in diamond–Origin of the brown coloration? Diam. Relat. Mater. 2009, 18, 843–845. [CrossRef] 3. Mäki, J.M.; Tuomisto, F.; Kelly, C.J.; Fisher, D.; Martineau, P.M. Properties of optically active vacancy clusters in type IIa diamond. J. Phys. Condens. Matter 2009, 21, 364216–364225. [CrossRef][PubMed] 4. Jones, R. Dislocations, vacancies and the brown color of CVD and natural diamond. Diam. Relat. Mater. 2009, 18, 820–826. [CrossRef] 5. Guagliardo, P.; Byrne, K.; Chapman, J.; Sudarshan, K.; Samarin, S.; Williams, J. Positron annihilation and optical studies of natural brown type I diamonds. Diam. Relat. Mater. 2013, 37, 37–40. [CrossRef] 6. Hainschwang, T. Classification and Color Origin of Brown Diamonds; Université de Nantes: Nantes, France, 2003. 7. Massi, L. Étude des défauts dans les diamants bruns et les diamants riches en hydrogène. Ph.D. Thesis, University of Nantes, Nantes, France, 2006. 8. Massi, L.; Fritsch, E.; Collins, A.T.; Hainschwang, T.; Notari, F. The “amber centers” and their relation to the brown color in diamond. Diam. Relat. Mater. 2005, 14, 1623–1629. [CrossRef] 9. Hainschwang, T.; Respinger, A.; Notari, F.; Hartmann, H.J.; Günthard, C. A comparison of diamonds irradiated by high fluence neutrons or electrons, before and after annealing. Diam. Relat. Mater. 2009, 18, 1223–1234. [CrossRef] 10. Hainschwang, T.; Pamies, G. Diamond Treatments Revisited: A Study of The Behaviour of Diamonds and Their Defects When Exposed to Irradiation, Annealing and HPHT Technique; University of Warwick: Coventry, UK, 2019. 11. Dobrinets, I.A.; Vins, V.G.; Zaitsev, A.M. HPHT-Treated Diamonds Springer-Verlag Berlin. Springer Ser. Mater. Sci. 2016, 181, 257. Minerals 2020, 10, 903 38 of 38

12. Collins, A.T.; Kanda, H.; Kitawaki, H. Color changes produced in natural brown diamonds by high-pressure, high-temperature treatment. Diam. Relat. Mater. 2000, 9, 113–122. [CrossRef] 13. Hainschwang, T.; Fritsch, E.; Notari, F.; Rondeau, B.; Katrusha, A. The origin of color in natural C center bearing diamonds. Diam. Relat. Mater. 2013, 39, 27–40. [CrossRef] 14. Hainschwang, T.; Notari, F.; Fritsch, E.; Massi, L.; Rondeau, B.; Breeding, M.; Vollstaedt, H. HPHT treatment

of CO2 containing and CO2—Related brown diamonds. Diam. Relat. Mater. 2008, 17, 340–351. [CrossRef] 15. Hainschwang, T. CVD Synthetic Diamonds Identiﬁed in a Parcel of Light Brown Melee. J. Gemmol. 2020, 37, 16–18. [CrossRef] 16. Field, J.E. The Properties of Natural and Synthetic Diamond; Academic Press: London, UK, 1992. 17. Hainschwang, T.; Fritsch, E.; Massi, L.; Rondeau, B.; Notari, F. The C center isolated nitrogen-related infrared 1 absorption at 2688 cm− : Perfect harmony in diamond. J. Appl. Spectrosc. 2012, 79, 749–754. [CrossRef] 18. Hainschwang, T.; Fritsch, E.; Notari, F.; Rondeau, B. A new defect center in type Ib diamond inducing one phonon infrared absorption: The Y center. Diam. Relat. Mater. 2012, 21, 120–126. [CrossRef] 19. Hainschwang, T. Diamants de type Ib: Relations entre les propriétés physiques et gemmologiques des diamants contenant de l’azote isolé. Ph.D. Thesis, University of Nantes, Nantes, France, 2014. 20. Fritsch, E.; Hainschwang, T.; Massi, L.; Rondeau, B. Hydrogen-related optical centers in natural diamond: An update. New Diam. Front. Carbon Technol. 2007, 17, 63–89. 21. Collins, A.T.; Davies, G.; Woods, G.S. Spectroscopic studies of the H1b and H1c absorption lines in irradiated, annealed type-Ia diamonds. J. Phys. C Solid State Phys. 1986, 19, 3933–3944. [CrossRef] 22. Eaton-Magaña, S.; Ardon, T.; Zaitsev, A.M. LPHT annealing of brown-to-yellow type Ia diamonds. Diam. Relat. Mater. 2017, 77, 159–170. [CrossRef] 23. Collins, A.T. Vacancy enhanced aggregation of nitrogen in diamond. J. Phys. C Solid State Phys. 1980, 13, 2641–2650. [CrossRef] 24. De Weerdt, F.; Collins, A.T. Broad-band luminescence in natural brown type Ia diamonds. Diam. Relat. Mater. 2007, 16, 512–516. [CrossRef] 25. Harlow, G.E. The Nature of Diamonds; Cambridge University Press: Cambridge, UK, 1998. 26. Eaton-Magaña, S.; Ardon, T.; Smit, K.V.; Breeding, C.M.; Shigley, J.E. Natural-color pink, purple, red and brown diamonds: Band of many colors. Gems. Gemol. 2018, 54, 352–377. 27. Nazaré, M.H.; Neves, A.J.; Davies, G. Optical studies of the 1.40-eV Ni center in diamond. Phys. Rev. B 1991, 43, 14196–14205. [CrossRef][PubMed] 28. Gippius, A.A. Luminescent characterization of radiation damage and impurities in ion-implanted natural diamond. Diam. Relat. Mater. 1993, 2, 640–645. [CrossRef] 29. Thomaz, M.F.; Davies, G. The decay time of N3 luminescence in natural diamond. Proc. Roy. Soc. Lond. Ser. A 1978, 362, 405–419. 30. Vasil’ev, E.A.; Ivanov-Omskii, V.I.; Pomazanskii, B.S.; Bogush, I.N. The N3 center luminescence quenched by nitrogen impurity in natural diamond. Tech. Phys. Lett. 2004, 30, 802–803. [CrossRef] 31. Katrusha, A.; V.N. Bakul Institute for Superhard Materials, National Academy of Sciences of Ukraine, Kiev, Ukraine. HPHT Processing of Brown to Pink Diamonds. Personal communication, 2020. 32. Yuryeva, O.P.; Rakhmanova, M.I.; Zedgenizov, D.A.; Kalinina, V.V. Spectroscopic evidence of the origin of brown and pink diamonds family from Internatsionalnaya kimberlite pipe (Siberian craton). Phys. Chem. Miner. 2020, 47, 1–19. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).