Raman and Photoluminescence Mapping of Gem Materials

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Article Raman and Photoluminescence Mapping of Gem Materials

Sally Eaton-Magaña *, Christopher M. Breeding, Aaron C. Palke, Artitaya Homkrajae, Ziyin Sun and Garrett McElhenny

Gemological Institute of America, Carlsbad, CA 92008, USA; [email protected] (C.M.B.); [email protected] (A.C.P.); [email protected] (A.H.); [email protected] (Z.S.); [email protected] (G.M.) * Correspondence: [email protected]

Abstract: Raman and photoluminescence (PL) mapping is a non-destructive method which allows gemologists and scientists to evaluate the spatial distributions of defects within a gem; it can also provide a method to quickly distinguish different species within a composite gem. This article provides a summary of this relatively new technology and its instrumentation. Additionally, we provide a compilation of new data for various applications on several gemstones. Spatial differences within diamonds can be explored using PL mapping, such as radiation stains observed on the rough surface of natural green diamonds. Raman mapping has proven useful in distinguishing between omphacite and jadeite within a composite of these two minerals, identifying various tourmaline species within a heterogeneous mixture, and determining the calcium carbonate polymorphs in pearls. Additionally, it has potential to be useful for country-of-origin determination in blue sapphires and micro-inclusion analysis. As new avenues of research are explored, more applications for gem materials will inevitably be discovered.

Keywords: Raman mapping; photoluminescence mapping; spectroscopy; diamond; corundum; tourmaline; jadeite; pearl; gemology Citation: Eaton-Magaña, S.; Breeding, C.M.; Palke, A.C.; Homkrajae, A.; Sun, Z.; McElhenny, G. Raman and Photoluminescence Mapping of Gem Materials. Minerals 1. Introduction 2021, 11, 177. https://doi.org/ Gemological laboratories rely on non-destructive analytical techniques that are gen- 10.3390/min11020177 erally based on optical methods such as absorption spectroscopy along with Raman and photoluminescence (PL) spectroscopy. PL and Raman spectroscopy, as used for gemstones, Academic Editor: is a microscope-assisted analytical technique in which a material is illuminated with laser Thomas Hainschwang light and the resulting emission is measured with a high-resolution spectrometer. Received: 12 January 2021 PL spectroscopy has become an indispensable tool used by major gemological labora- Accepted: 1 February 2021 tories to distinguish treated and lab-grown diamonds from their natural counterparts [1,2]. Published: 8 February 2021 Within diamond, the presence, and thus the detection, of various defects differs with its growth history along with any subsequent treatment. PL spectroscopy can detect these Publisher’s Note: MDPI stays neutral features even at concentrations lower than 10 ppb [3]. Raman spectroscopy has proven with regard to jurisdictional claims in vital for a host of other gemstones as well to aid in their identification [4–7]. published maps and institutional affil- Raman and PL mapping is a logical extension of these methods in which spectra iations. can be collected quickly and automatically across an area instead of a single point (or the sample manually moved to collect multiple spectra across a sample). These mapping techniques have become possible in the past few years as the technology for the necessary instrumentation has improved. Therefore, Raman and PL mapping have become ideal Copyright: © 2021 by the authors. methods to analyze the spatial differences in these diamonds and similar diamonds that are Licensee MDPI, Basel, Switzerland. distinguished by distinct growth regions. This new instrumentation automatically collects This article is an open access article hundreds to thousands of spectra across a sample and has provided several new, exciting distributed under the terms and research opportunities and identification avenues in recent years. conditions of the Creative Commons This article provides a brief account of investigations across a broad range of research Attribution (CC BY) license (https:// and identification interests for a wide array of different gemstones. It offers an indication creativecommons.org/licenses/by/ 4.0/). of the variety of research possibilities across the breadth of the gemological world.

Minerals 2021, 11, 177. https://doi.org/10.3390/min11020177 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 34

This article provides a brief account of investigations across a broad range of research and identification interests for a wide array of different gemstones. It offers an indication Minerals 2021, 11, 177 2 of 31 of the variety of research possibilities across the breadth of the gemological world.

2. Materials and Methods 2. MaterialsGenerally, and Raman/PL Methods mapping is used to show the distribution of the peak intensity associatedGenerally, with the Raman/PL concentration mapping of an is optical used to defect; show thehowever distribution, other ofdata the such peak as intensity peak positionassociated or peak with width the concentration can be plotted of instead, an optical depending defect; however,on the application. other data A such Raman/PL as peak mappingposition microscope or peak width can can collect be plotted thousands instead, of dependingspectra in a on raster the application. pattern (Figure A Raman/PL 1) and recordmapping a spectrum microscope at each can point. collect The thousands collection ofarea spectra on the in gem’s a raster surface pattern for (Figureeach individ-1) and ualrecord spectrum a spectrum can be at quite each point.small (<1 The µ collectionm2) or quite area large on the (>1000 gem’s µm surface2) if needed for each to individual accom- 2 2 modatespectrum time can constraints be quite small or a (<1 veryµm weak) or quiteemission large signal. (>1000 Theµm )spectral if needed data to accommodate(typically a usertime-specified constraints peak or selected a very from weak the emission spectrum) signal. are then The interpolated spectral data to (typicallyproduce a a map user- ofspeciﬁed a defect’s peak distribution. selected fromWith thestandard spectrum) data collection, are then interpolated a single location to produce (represented a map by of a thedefect’s green distribution.circle) is chosen With for standard data collection data collection, which agenerally single location serves (representedas a proxy for by an the entiregreen sample circle) (although is chosen fora researcher data collection can collect which from generally multiple serves spots as on a a proxy sample for to an gauge entire thesample heterogeneity (although). With a researcher mapping can spectroscopy, collect from the multiple data collection spots on is a automated sample to such gauge that the dataheterogeneity). from the entire With surfacemapping may spectroscopy, be collected the anddata stitched collection together is automated, yielding such a that much data greaterfrom theand entire more surface complete may image be collected of the defects and stitched within together, the sample yielding and generally a much greater offering and thousandsmore complete of collected image spectra of the defects instead within of a single the sample spectrum. and generally offering thousands of collected spectra instead of a single spectrum.

FigureFigure 1. 1.DataData collection collection across across the the table table facet facet of of a around round diamond diamond with with standard standard spectroscopy (Left) (Leftis compared) is compared against against mapping mapping spectroscopy spectroscopy in which in which spectra spectra are collectedare collected across across the entirethe entire surface surface(Right ()[Right8]. ) [8].

SomeSome of of the the considerations considerations that that we we encountered encountered during during the the proc processess of of data data collection collection includedincluded balancing balancing the the collection collection time time against against data data quality quality,, mitigatingmitigating thethe effectseffects of of internal inter- nalreflections, reflections, and and ensuring ensuring that that the the plane plane of of data data collection collection is properlyis properly leveled. leveled. MostMost gemstones gemstones cannot cannot be be cooled cooled to to liquid liquid nitrogen nitrogen temperatures temperatures to to optimize optimize the the resultsresults from from PL PL spectroscopy. spectroscopy. Diamonds Diamonds are are the the exception exception as as they they have have a alow low thermal thermal expansionexpansion coefficient coefficient and and extremely extremely high high thermal thermal conductivity, conductivity, so so they they can can be be cool cooleded to to lowlow temperatures. temperatures. Other Other gems, gems, if ifcooled, cooled, have have a amuch much higher higher risk risk of of fracture. fracture. For For example, example, thethe coefficient coefficient of of thermal thermal expansion expansion of of corundum corundum is is five five times times greater greater [9] [9 and] and the the thermal thermal conductivityconductivity is isat at least least 65 65 times times lower lower than than diamond diamond [10] [10. ]. Therefore,Therefore, since since diamonds diamonds can can be be cooled cooled,, PL PL maps maps are are typically typically conducted conducted with with the the ◦ stonestone immersed immersed in in liquid liquid nitrogen nitrogen (− (196−196 °C)C),, although although using using a cryostat a cryostat is isalso also a possibility a possibility.. PLPL peaks peaks of of various various defects defects in in diamond diamond tend tend to to be be sharper, sharper, and and thus thus appear appear more more intense, intense, atat liquid liquid nitrogen nitrogen temperatures temperatures [11,12 [11,12]. ].With With our our current current experimental experimental setup, setup, the the possible possible timetime window window before before nitrogen nitrogen boil boil-off-off is islimited limited to to around around 10 10 m min,in, which which is is achieved achieved by by choosingchoosing an an exposure exposure time, time, co collectionllection area, area, and and pixel pixel size size such such that that the the collection collection is is within within that time frame. For Raman and PL spectroscopy conducted for other gemstones at room that time frame. For Raman and PL spectroscopy conducted for other gemstones at room temperature, there are generally not the same time constraints and, if feasible, maps can be temperature, there are generally not the same time constraints and, if feasible, maps can collected over many hours. be collected over many hours. Additionally, better results are obtained for diamond samples if, prior to mapping, a temporary coating of Pelco colloidal graphite is painted onto the bottom side of the sample. This coating can reduce the amount of light reflecting off other facets and returning to the table (Figure2). This coating is easily removed after testing with ethanol. This step is generally not used with other gemstones. Minerals 2021, 11, x FOR PEER REVIEW 3 of 34

Additionally, better results are obtained for diamond samples if, prior to mapping, a temporary coating of Pelco colloidal graphite is painted onto the bottom side of the sam- Minerals 2021, 11, 177 ple. This coating can reduce the amount of light reflecting off other facets and returning3 of 31 to the table (Figure 2). This coating is easily removed after testing with ethanol. This step is generally not used with other gemstones.

Figure 2. These PL maps were collected on the table of a faceted diamond at liquid nitrogen temperature. The PL map on Figure 2. These PL maps were collected on the table of a faceted diamond at liquid nitrogen temperature. The PL map on the left was collected without any graphite coating on the pavilion faceted while the PL map at right had the coating to the left was collected without any graphite coating on the pavilion faceted while the PL map at right had the coating to reduce the pavilion reflections. reduce the pavilion reflections. While some researc researchershers prefer to collect PL/Raman mapping mapping data data from from flat flat plate plate sam- sam- ples only, this article relates data for several gemological applications and research studies thatthat were were obtained obtained from from faceted or rough rough samples. All the ma mappingpping studies studies discussed here and used in prior studies at the Gemological Institute of America ( (GIA)GIA) (e.g., [8,13 [8,13–15]])) were performed using a Thermo Scientific Scientific DXRxi Raman imaging microscope ( (Madison,Madison, WWI,I, USA USA)) with 455, 532,532, 633, and 780 nm laser excitationexcitation wavelengths. The DXRxi has an Olympus optical microscope and an Andor Andor Technology Technology Newton 970 electron electron-multiplying-multiplying chargecharge-coupled-coupled device (EMCCD).(EMCCD). AnAn OlympusOlympus (either(either 10 10××;; NANA == 0.25 oror 2020××;; NA NA = 0.40 0.40)) objective lens was used. The DXRxi uses a continuously moving, variable speed sample stagestage driven by linear magnetic motor motors.s. The stage movement is is synchronized synchronized with with the the EMCCD EMCCD detector detector so so that that the the repeata- repeata- bibilitylity of of spectra spectra collection collection at at the the intended intended location location positions positions is within is within 100 100nm nm[8]. The [8]. data The weredata wereprocessed processed using using Thermo Thermo Scientific’s Scientific’s OMNICxi OMNICxi analysis analysis software software package, package, and base- and linebaseline-corrected-corrected peak peakarea profiles area profiles were used were to used produce to produce the observed the observed PL maps. PL The maps. spectr Theal dataspectral can dataalso be can exported also be exported to external to externalsoftware. software. The user Theis able user to isdefine able towhether define whetherthe data arethe collected data are collectedin confocal in or confocal standard or standardmode, along mode, with along the pixel with size the pixel(size sizealong (size one along edge one edge of the collection area square), number of scans, collection time per spectrum of the collection area square), number of scans, collection time per spectrum (typically << (typically << 1 s), and the specific collection area (typically after a visual image of the 1 s), and the specific collection area (typically after a visual image of the sample has been sample has been collected by the instrument). collected by the instrument). The collection of thousands of spectra requires the researcher to be able to process The collection of thousands of spectra requires the researcher to be able to process the data in a meaningful way for interpretation. The collected data are often visualized the data in a meaningful way for interpretation. The collected data are often visualized using false color maps of specific peaks or spectral features. As means of illustration, a full using false color maps of specific peaks or spectral features. As means of illustration, a full complement of data is shown for a diamond with pronounced brown graining (Figure3A ). complement of data is shown for a diamond with pronounced brown graining (Figure This diamond was immersed in liquid nitrogen and PL maps, each with thousands of 3A). This diamond was immersed in liquid nitrogen and PL maps, each with thousands spectra (Figure3B), were collected with all available lasers. False color maps could be of spectra (Figure 3B), were collected with all available lasers. False color maps could be generated for each of the detected defects (e.g., Figure3C). A few features detected within generated for each of the detected defects (e.g., Figure 3C). A few features detected within0 the spectra are plotted here, such as normalized peak area intensity of the H4 defect [N4V2] theat 495.9 spectra and are at 490.7 plotted nm—an here, uncharacterizedsuch as normalized feature peak often area seen intensity in natural of diamonds the H4 defect [16], 0 [Nparticularly4V2] at 495.9 in brown and at type 490.7 Ia diamonds,nm—an uncharacterized where it often occursfeature in often conjunction seen in withnatural H4 [dia-17]. mondsAs standard [16], particularly practice with in diamond brown type samples, Ia diamonds the calculated, where area it often of the occurs peak in area conjunction is ratioed withto the H4 peak [17] area. As ofstandard the diamond practice Raman with diamond peak, providing samples an, the internal calculated normalization area of the of peak the areapeak is areas. ratioed Therefore, to the peak care area is taken of the to ensurediamond that Raman these peakspeak, providing are unsaturated an internal across nor- the malizationcollection area; of the see peak Figure areas3B.. Also Therefore, seen is thecare full is widthtaken to at half-maximum—theensure that these peaks peak a widthre un- of the H3 defect [NVN]0 at 503.2 nm illustrating that several types of spectral features can be chronicled and evaluated. The data can also be exported to other software for additional analysis of the possible interrelationships between defects (Figure3E). Minerals 2021, 11, 177 4 of 31

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Figure 3. (A) HereFigure is an 3.example(A) Here of a is type an example Ia natural of diamond a type Ia with natural pronounced diamond brown with graining. pronounced (B) A brown few individual graining. spectra along with(B) some A few example individual calculations spectra alongare shown with as some illustration example of calculations the internal are, baseline shown-corrected as illustration calculations of the occurring for the dozens of possible variables across the thousands of collected spectra. (C) These false color PL maps were internal, baseline-corrected calculations occurring for the dozens of possible variables across the assembled from data collected with 455 nm excitation with the diamond submerged in liquid nitrogen. The PL mapping data are composedthousands of ~26,000 of individual collected spectra spectra. taken (C) Thesewith a false10 µm color pixel PLsize maps in 4½ were min. assembledThey show the from normalized data collected peak intensity for the withH4 defect 455 nmat 495.9 excitation nm and with the thePL feature diamond at 490.7 submerged nm. (D) inFor liquid the PL nitrogen. map shown The in PL Figure mapping 3C, right, data the are color ranges are adjusted to opposite extremes to illustrate the differences in data representation1 that can be seen from composed of ~26,000 individual spectra taken with a 10 µm pixel size in 4 2 min. They show the adjusting the colornormalized limits. (E) peakAdditional intensity analysis for the of H4the defectspectral at data 495.9 is nmpossible andthe by PLexporting feature the at 490.7calculated nm. (valuesD) For into the external programming, such as plotting the intensities of various spectral features within a spreadsheet software in which PL map shown in Figure3C, right, the color ranges are adjusted to opposite extremes to illustrate the each datapoint originates from a different spectrum. The data shown here for H3 and the 490.7 nm defect are for illustration only; no correlationdifferences can be in concluded data representation based on results that can from be a seensingle from sample. adjusting the color limits. (E) Additional analysis of the spectral data is possible by exporting the calculated values into external programming, such as plotting the intensities of various spectral features within a spreadsheet software in which each datapoint originates from a different spectrum. The data shown here for H3 and the 490.7 nm defect are for illustration only; no correlation can be concluded based on results from a single sample. Minerals 2021, 11, 177 5 of 31

Another important consideration is proper data presentation and taking care that the false color maps accurately represent the data. Depending on the limits chosen, the range of data can be perceived very differently by the reader (compare Figure3C, right with Figure3D). Additionally, the rainbow color palette is commonly used in many software programs (including the one accompanying the OMNICxi used in these studies), but it can be problematic as the information can be perceived quite differently if reproduced in gray-scale or observed by individuals with color vision deﬁciencies [18]. With that in mind, we have modiﬁed the color palette used in these Raman/PL maps to account for such issues facing false color maps. With all the advances in instrumentation in recent years, Raman and PL maps can be created in a few minutes and create the ability to evaluate the distribution of defects and optical features in gemstones.

3. Distinction between Raman Shift and Photoluminescence

Minerals 2021, 11, x FOR PEER REVIEW In this article, we will describe mapping of both Raman spectra6 of 34 and PL spectra so we wanted to provide a brief distinction between these two measurements as they are collected with this same instrumentation. PL and Raman peaks are detected with the same instrumentation,3. Distinction between and Raman both Shift types and Photoluminescence of peaks appear in the same spectra [12]. Luminescence peaksIn this are article, emitted we will at describe a consistent mapping energy of both (orRaman wavelength) spectra and PL from spectra a so material. we For example, the wanted to provide a brief distinction between these two measurements as they are col- chromium-relatedlected with this same instrumentation. ﬂuorescence PL peaksand Raman in ruby peaks occurare detected at 693 with and the 694same nm; the luminescence willinstrumentation, not shift toand a both different types of wavelengthpeaks appear in ifthe a same different spectra light [12]. Luminescence source is used [11]. peaks Inare contrast,emitted at a Ramanconsistent peaksenergy (or have wavelength) a constant from a energy material. differenceFor example, from their excitation source.the chromium The-related source fluorescence laser interacts peaks in ruby with occur the at molecular693 and 694 nm vibrations; the lumines- within the gem, which cence will not shift to a different wavelength if a different light source is used [11]. producesIn contrast, a change Raman peaks when have the a constant light is energy re-emitted difference by from the their gem excitation [19]. Raman spectroscopy measuressource. The source the energy laser interacts shift causedwith the bymolecular these vibrations molecular within interactions, the gem, which which occurs in only one inproduces 100 million a change photons. when the light is re-emitted by the gem [19]. Raman spectroscopy measures the energy shift caused by these molecular interactions, which occurs in only one in While100 million PL photons. peaks have ﬁxed energies and wavelengths (typically given in nanome- tersWhile in the PL peaks gemological have fixed energies literature), and wavelengths Raman shift(typically values given in are nanometers usually reported in units of wavenumbers,in the gemological literature), cm–1, and Raman the shift wavelength values are usua oflly the reported laser in set units as of the wave- “zero” wavenumber po- numbers, cm–1, and the wavelength of the laser set as the “zero” wavenumber position. In sition.Raman spectra, In Raman the reported spectra, values the arereported relative to the values excitation are source relative and are to generally the excitation source and are generallylabeled as Raman labeled shift values. as Raman When Raman shift values. peaks are Whenshown in Raman a PL spectrum, peaks the are wave- shown in a PL spectrum, thelength wavelength position is dependent position on the is excitation dependent frequency on the while excitation the PL peaks frequency are independ- while the PL peaks are independentent (Figure 4). (Figure4).

Figure 4. 4.PL PLspectra spectra were collected were collected on the same on diamond the same using diamond two different using lasers two(488 and different 514 lasers (488 and 514 nm) nm) at liquid nitrogen temperature. The resulting spectra are plotted against the wavelength. Each atlaser liquid shows a nitrogen different location temperature. for the diamond The Raman resulting line based spectra on the excitation are plotted wavelength. against the wavelength. Each laserFor both shows lasers, the a differentenergy difference location between for the the excitation diamond wavelength Raman and linethe Raman based line on is the excitation wavelength. constant—1332 cm−1. Each spectrum shows the luminescence feature NV0 and the activation for its Forzero-phonon bothlasers, line (ZPL) the is fixed energy at 575 differencenm regardlessbetween of the excitation the source. excitation wavelength and the Raman line is constant—1332 cm−1. Each spectrum shows the luminescence feature NV0 and the activation for its zero-phonon4. PL and Raman line Mapping (ZPL) of is Natural ﬁxed at Diamonds 575 nm regardlesswith Color Zoning of the excitation source. 4.1. Natural Pink Diamond PL mapping has shown research potential regarding natural diamonds, particularly in those that show spatial differences in their color. One such example is type Ia pink diamond [20], in which the color is concentrated into thin lamellae (Figure 5A). These pink Minerals 2021, 11, 177 6 of 31

4. PL and Raman Mapping of Natural Diamonds with Color Zoning Minerals 2021, 11, x FOR PEER REVIEW4.1. Natural Pink Diamond 7 of 34

PL mapping has shown research potential regarding natural diamonds, particularly in those that show spatial differences in their color. One such example is type Ia pink lamellaediamond can [be20 seen], in within which a the microscope color is concentratedand explored in into more thin depth lamellae with (Figuremapping5A). spec- These troscopypink lamellae [13]. These can colored be seen lamellae within a are microscope caused by and natural explored plastic in moredeformation depth with, oriented mapping alongspectroscopy the (111) crystallographic [13]. These colored planes, lamellae and create are a caused broad absorptio by naturaln band plastic centered deformation, at ~550oriented nm. There along is thealso (111)a corresponding crystallographic emission planes, band and at 600 create–750 anm broad [13].absorption Since much band aboutcentered the 550 at ~550nm absorption nm. There isband also in a correspondingnatural pink diamonds emission is band unknown, at 600–750 studying nm [13 the]. Since 600much–750 nm about emission the 550 band nm absorptionprovides opportunities band in natural to learn pink more diamonds about pink is unknown, diamonds. studying theIn 600–750 pink diamonds nm emission that are band type provides IaA>B, the opportunities pink color is to typically learn more only about seen pinkwithin diamonds. the lamellae,In which pink diamonds are very straight that are and type parallel IaA>B, [20,21 the pink]. Similarly, color is the typically normalized only seenintegrated within the intensitylamellae, of the which ~600 are–750 very nm straight emission and band parallel (area [ 20underneath,21]. Similarly, the emission the normalized band normal- integrated izedintensity using the of the diamond ~600–750 Raman nm emission peak) qualitatively band (area underneath corresponds the with emission the saturation band normalized of pinkusing color the (Figure diamond 5B,C). Raman With PL peak) mapping qualitatively of these diamonds corresponds, the with ~600 the–750 saturation nm emission of pink bandcolor is not (Figure detected5B,C). ou Withtside PL of mappingthe lamella of theseand has diamonds, calculated the areas ~600–750 of near nm zero. emission In con- band trast,is not within detected the lamellae outside, the of theemission lamella band and is hascomparatively calculated quite areas large. of near zero. In contrast, within the lamellae, the emission band is comparatively quite large.

FigureFigure 5. ( 5.A) (ThisA) This thin thinplate plate (4 × 4 (4 × 1.6× 4mm)× 1.6 fashioned mm) fashioned from a natural from adiamond natural sourced diamond from sourced Sibe- from riaSiberia showed showed pink-to pink-to-purple-purple coloration coloration concentrated concentrated along thin along parallel thin lamellae parallel lamellae[22]. (B) The [22]. PL (B ) The PL map of a broad emission band (~600–750 nm) using 532 nm excitation. (C) A line scan of a portion map of a broad emission band (~600–750 nm) using 532 nm excitation. (C) A line scan of a portion of of the map. The data were collected in confocal mode with a 3 µm pixel size. the map. The data were collected in confocal mode with a 3 µm pixel size. 4.2. Natural Diamonds from Marange with Hydrogen Clouds Another example of mapping on color-zoned natural diamonds is those from the Ma- range alluvial deposit in Zimbabwe. This is a consistent source of mixed-habit type IaAB Minerals 2021, 11, 177 7 of 31

Minerals 2021, 11, x FOR PEER REVIEW 8 of 34 4.2. Natural Diamonds from Marange with Hydrogen Clouds Another example of mapping on color-zoned natural diamonds is those from the Marange alluvial deposit in Zimbabwe. This is a consistent source of mixed-habit type diamonds that contain cuboid (grayish) and octahedral (colorless) sectors [23,24]. In these IaAB diamonds that contain cuboid (grayish) and octahedral (colorless) sectors [23,24]. diamonds, Raman mapping can be used to establish that the gray color is due to graphite In these diamonds, Raman mapping can be used to establish that the gray color is due micro-inclusions (Figure 6; [24–26]) along with the presence of CH4 [24]. In addition to to graphite micro-inclusions (Figure6;[ 24–26]) along with the presence of CH4 [24]. In Raman mapping, PL mapping of these regions also confirmed the incorporation of other addition to Raman mapping, PL mapping of these regions also conﬁrmed the incorporation defectsof other including defects including a variety aof variety nickel- ofrelated nickel-related centers, such centers, as the such S3 asdefect the S3and defect PL peaks and PLat 694peaks and at 700 694 nm and [23] 700. nm [23].

FigureFigure 6. 6. RamanRaman mapping mapping of ofa cubo a cubo-octahedral-octahedral diamond diamond plate plate from from the Marange the Marange deposit deposit in Zimbabwe in Zimbabwe,, which whichoften show oftens regionsshows regionsof grayish of grayishclouds. clouds.The image The and image Raman and map Raman are maplooking are at looking the boundary at the boundary between betweencolorless colorlessand included and includedregions, −1 studyingregions, studying the graphite the graphite-related-related Raman feature Raman featureat a Raman at a Ramanshift of shift 1600 of cm 1600. This cm− 1room. This-temperature room-temperature Raman Raman map is map com- is posed of 2968 individual spectra with 5 µm pixel size and took 4.5 h to complete using 532 nm excitation. composed of 2968 individual spectra with 5 µm pixel size and took 4.5 h to complete using 532 nm excitation. 5.5. Natural Naturallyly Irradiated Irradiated Diamond Diamond DiamondDiamond is is among among the the most most valued valued and and desired desired of of gemstones gemstones and and when when they they occur occur withwith vibrant vibrant colors colors such such as as red, red, blue, blue, or or green, green, their their value value skyrockets skyrockets due due to to the the geological geological rarityrarity of of the uniqueunique conditionsconditions required required to to produce produce the the atomic atomic structural structural defects defects responsible respon- siblefor those for those colors. colors. Unlike Unlike most most colored colored diamonds, diamonds, many many green green diamonds diamonds obtain obtain their colortheir colorin a secondaryin a secondary fashion fashion as they as they are are exposed exposed to directto direct contact contact with with radioactive radioactive minerals miner- alsand and ﬂuids fluids in in the the shallow shallow parts parts of of the the Earth’s Earth’s crust crust after after theythey havehave beenbeen transported up up fromfrom the the mantle mantle where where they they crystallized. crystallized. Very Very commonly commonly with with natural natural radiation radiation exposure exposure comescomes the the presence presence of of small small areas areas of ofintense intense alpha alpha particle particle radiati radiationon damage damage on the on sur- the facesurface of diamonds. of diamonds. These These green green-- or orbrown brown-colored-colored patches, patches, termed termed “radiation “radiation stains” stains” by by gemologists,gemologists, usually usually penetrate penetrate on onlyly a a few few tens tens of of microns microns into into the the diamond diamond surface surface (i.e., (i.e., thethe stopping stopping distance ofof alphaalpha particlesparticles in in diamond). diamond). While While many many natural natural diamond diamond crystals crys- talsshow show a few a few of theseof these radiation radiation stains, stains, only only a a rare rare few few are are exposedexposed toto enough radiation damagedamage to to produce produce internal internal color color zoning zoning (beta (beta or or gamma gamma radiation radiation-related)-related) in in addition addition to to thethe surficial surﬁcial stains stains [27] [27].. These These rare rare stones stones produce produce some some of of the the world’s world’s most most famous famous and and valuablevaluable green green diamonds diamonds,, including including the the 40.70 40.70 carat Dresden Green [28,29 [28,29]].. PLPL analysis analysis of of diamond diamond is isan an important important tool tool in identifying in identifying many many types types of defects of defects and treatmentsand treatments in gem in diamonds. gem diamonds. In addition In addition to the tonaturally the naturally irradiated irradiated diamonds diamonds described de- here,scribed PL here,mapping PL mapping can also canbe helpful also be for helpful analyzing for analyzing the spatial the effects spatial of effects laboratory of laboratory irradi- irradiation, such as depth penetration of the irradiation and its effect on PL-active de- ation, such as depth penetration of the irradiation and its effect on PL-active defects [14,30]. The surficial contact with radioactive fluids or mineral grains often creates distinct spatial differences in PL defects related to natural irradiation. These green radiation stains have also been shown to turn to brown when heated to moderate temperatures (~500 °C Minerals 2021, 11, 177 8 of 31

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fects [14,30]. The surﬁcial contact with radioactive ﬂuids or mineral grains often creates distinct spatial differences in PL defects related to natural irradiation. These green radiation stains[31,32] have) and also, occasionally been shown, green to turn and to brown brown radiation when heated stains to are moderate seen on temperatures the same dia- ◦ (~500mond.C[ PL31 mapping,32]) and, hasoccasionally, helped elucidate green and the brown differences radiation between stains are adjacent seen on green the same and diamond.brown radiation PL mapping stains has[33] helped. Several elucidate useful exam the differencesples of PL betweenon naturally adjacent irradiated green anddia- brownmond radiationare recounted stains here. [33]. Several useful examples of PL on naturally irradiated diamond are recounted here. 5.1. PL Mapping to Determine Penetration Depth of Radiation 5.1. PL Mapping to Determine Penetration Depth of Radiation One such method to monitor the spatial variations is by monitoring vacancy defects introducedOne such by methodnatural toradiation. monitor PL the mapping spatial variations of natural is diamonds by monitoring with vacancyradiation defects stains introduced by natural radiation. PL mapping of natural diamonds with radiation stains allows us to see the distribution of vacancy defects (GR1; [V0], ZPL = 741 nm) relative to allows us to see the distribution of vacancy defects (GR1; [V0], ZPL = 741 nm) relative to the stains to better understand the impact of natural geological processes. Interestingly, the stains to better understand the impact of natural geological processes. Interestingly, PL PL mapping shows that heavily irradiated areas that have dark green radiation stains on mapping shows that heavily irradiated areas that have dark green radiation stains on a a diamond surface, themselves, have little to no GR1 emission (Figure 7). This is likely due diamond surface, themselves, have little to no GR1 emission (Figure7). This is likely due to the severe damaging of the diamond atomic lattice at the spot of most intense radiation to the severe damaging of the diamond atomic lattice at the spot of most intense radiation dose, rather than an absence of vacancies. Adjacent to the radiation stains, mapping shows dose, rather than an absence of vacancies. Adjacent to the radiation stains, mapping shows narrow regions showing elevated concentrations of GR1 defects. These regions range in narrow regions showing elevated concentrations of GR1 defects. These regions range in width from 18.5 to 22 µm that coincide with a lighter green in color; this observation is width from 18.5 to 22 µm that coincide with a lighter green in color; this observation is consistent with the penetration depth for alpha irradiation [31,32]. consistent with the penetration depth for alpha irradiation [31,32].

FigureFigure 7.7.( (LeftLeft)) PLPL mappingmapping ofof GR1GR1 (vacancy)(vacancy) defectsdefects onon thethe surfacesurface ofof aa naturalnatural roughrough diamonddiamond on on heavily heavily irradiated irradiated areasareas that that show show dark dark green green stains stains and and ~20 ~20µm µm narrow narrow adjacent adjacent zones zones of GR1.of GR1. (Right (Right) A) higher A higher magniﬁcation magnification version version of the of leftthe imageleft image shows shows an overlay an overlay of the of regions the regions with highwith GR1high (boxGR1 with(box redwith outlines) red outlines) along a withlong the with corresponding the corresponding PL map, PL map, which is displaced in order to show the radiation features underneath. The arrows are indicating a few examples of which is displaced in order to show the radiation features underneath. The arrows are indicating a few examples of the the narrow regions with elevated GR1 concentration and have a width of 18.5–22 µm, consistent with the penetration narrow regions with elevated GR1 concentration and have a width of 18.5–22 µm, consistent with the penetration depth for depth for alpha irradiation. alpha irradiation. InIn additionaddition to to a a narrow narrow vacancy vacancy distribution distribution lateral lateral to radiation to radiation stains, stains, the depth the depth pen- penetrationetration within within a diamond a diamond is similar. is similar. From From a cleaved a cleaved diamond diamond sample sample with withradiation radiation dam- damageage and andgreen green stains stains at the atsurface, the surface, the depth the penetration depth penetration of both ofthe both stains the and stains resulting and resultingcolor zonation color zonation can clearly can be clearly seen be (Figure seen (Figure 8). It is8). difficult It is difﬁcult to obtain to obtain a cross a cross-section-section of a ofdiamond a diamond sample sample like like this this without without laser laser cutting cutting and and polishing. polishing. Radiation Radiation-related-related defectsdefects inin diamonddiamond areare veryvery sensitivesensitive toto temperaturetemperature andand thethe inadvertentinadvertent heatingheating associatedassociated with with cuttingcutting processesprocesses cancan oftenoften modifymodify thethe defects.defects. UsingUsing aa cleavedcleaved samplesample ensuredensured thatthat thethe defectsdefects remainedremained intact.intact. RamanRaman mappingmapping ofof GR1GR1 alongalong thethe edgeedge ofof thethe cleavedcleaved samplesample showedshowed twotwo distinctdistinct zones;zones; thethe oneone closestclosest toto thethe stainstain measuredmeasured 17.617.6µ µmm andand aa narrowernarrower transitionaltransitional zonezone waswas 3.93.9 µµm.m. Together,Together, the thetwo twozones zonestotaled totaled21.5 21.5µ µm,m,which whichis is consistentconsistent with the penetration of alpha damage in diamond that was also seen in Figure 7 for the surface of a natural diamond. Minerals 2021, 11, 177 9 of 31

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with the penetration of alpha damage in diamond that was also seen in Figure7 for the surface of a natural diamond.

FigureFigure 8. 8. PLPL mapping mapping of of GR1 GR1 at at the the edge edge of of a acleaved cleaved diamond diamond sample sample with with radiation radiation damage damage shows shows that that the the structure structure of of vacancyvacancy distribution distribution at at depth depth in in the the diamond diamond is is similar similar to to that that observed observed adjacent adjacent to to surface surface stains. stains.

ThroughThrough PL PL mapping, mapping, the the lateral lateral and and depth depth distribu distributionstions of of vacancy vacancy defects defects in in natu- natu- µ rallyrally irradiated irradiated diamonds diamonds with with green green radiation radiation stains stains are are both both shown shown to to comprise comprise ~20 ~20 µmm narrownarrow regions regions adjacent adjacent to to the the stains, stains, supporting supporting the the idea idea that that the the stains stains are are a aproduct product of of alphaalpha irradiation. irradiation. This This type type of of analysis analysis hel helpsps us us to to better better understand understand that that diamond diamond defects defects areare often often not not uniformly uniformly distributed andand cancan be be directly directly attributed attributed to to visible visible natural natural features. fea- Another example of PL mapping aiding in the analysis of natural-irradiation related tures. features was a faceted diamond that showed green fluorescence around the rim of some Another example of PL mapping aiding in the analysis of natural-irradiation related cavities present in the diamond [34]. Figure9 shows that, while in the Earth’s crust, the features was a faceted diamond that showed green fluorescence around the rim of some diamond was exposed to radioactive fluids, particularly in etch channels now in the form of cavities present in the diamond [34]. Figure 9 shows that, while in the Earth’s crust, the cavities on the table and crown facets. These were first noticed from fluorescence imaging diamond was exposed to radioactive fluids, particularly in etch channels now in the form that revealed green halos around these cavities. Although these isolated areas of fluores- of cavities on the table and crown facets. These were first noticed from fluorescence imag- cence around the cavities indicated that they had likely been filled with a radioactive fluid, ing that revealed green halos around these cavities. Although these isolated areas of fluo- there was none of the greenish color or radiation staining that would likely accompany rescence around the cavities indicated that they had likely been filled with a radioactive higher radiation doses. fluid, there was none of the greenish color or radiation staining that would likely accom- PL mapping of one of the cavities showed much higher intensities of the nitrogen- pany higher radiation doses. vacancy centers and the GR1. The extent of the fluorescence halos and the elevated GR1 is approximatelyPL mapping of 30 oneµm; of again,the cavities this isshowed consistent much with higher the penetrationintensities of depth the nitrogen of alpha- vacancyradiation. centers and the GR1. The extent of the fluorescence halos and the elevated GR1 is approximately 30 µm; again, this is consistent with the penetration depth of alpha radiation.5.2. PL Mapping Provides Clues to Unusual Origin Story The “Matryoshka” diamond is a 0.62 ct greenish rough diamond with a freely moving diamond trapped inside [35]. The exterior and interior diamonds both showed visual evidence of radiation stains and these observations were confirmed by PL mapping using 633 nm excitation and plotting the GR1 feature [V0] at 741.2 nm. Figure 10A shows a visual camera mosaic generated by the mapping instrument, indicating the internal diamond in focus and the external diamond out of focus. The PL maps were collected in confocal mode to best distinguish the PL spectra from the inner and outer diamond. Figure 10B shows the PL map of the GR1 defect. It shows generally a higher intensity of GR1 in the outer diamond, yet shows some high GR1 emission detected in some portions of the inner diamond (indicated by red arrow in Figure 10). This confirms that radioactive fluids penetrated inside the cavity and came into physical contact with the internal diamond

Minerals 2021, 11, x FOR PEER REVIEW 10 of 34

Figure 8. PL mapping of GR1 at the edge of a cleaved diamond sample with radiation damage shows that the structure of vacancy distribution at depth in the diamond is similar to that observed adjacent to surface stains.

Through PL mapping, the lateral and depth distributions of vacancy defects in naturally irradiated diamonds with green radiation stains are both shown to comprise ~20 µm narrow regions adjacent to the stains, supporting the idea that the stains are a product of alpha irradiation. This type of analysis helps us to better understand that diamond defects are often not uniformly distributed and can be directly attributed to visible natural features. Another example of PL mapping aiding in the analysis of natural-irradiation related features was a faceted diamond that showed green fluorescence around the rim of some cavities present in the diamond [34]. Figure 9 shows that, while in the Earth’s crust, the diamond was exposed to radioactive fluids, particularly in etch channels now in the form of cavities on the table and crown facets. These were first noticed from fluorescence imaging that revealed green halos around these cavities. Although these isolated areas of fluorescence around the cavities indicated that they had likely been filled with a radioactive fluid, there was none of the greenish color or radiation staining that would likely accom- Minerals 2021, 11, 177 10 of 31 pany higher radiation doses. PL mapping of one of the cavities showed much higher intensities of the nitrogen- vacancy centers and the GR1. The extent of the fluorescence halos and the elevated GR1 Minerals 2021, 11, x FOR PEER REVIEWiscrystal. approximately This type 30 of µm; analysis again, would this is have consistent been far with more the difﬁcult penetration to achieve depth withof alpha standard11 radi- of 34

ation.PL spectroscopy.

Figure 9. (Left) DiamondView fluorescence image showed interesting green fluorescence around the cavities on the crown facet of a 0.70 ct E-color diamond. The green fluorescent haloes were created by radioactive fluids (Center). Subsequent PL mapping with 532 nm excitation showed that high amounts of radiation brought about a number of localized changes in the diamond surrounding the cavities. (Right) A line scan (indicated by a black line in the central image) shows the normalized peak intensity for the GR1 from the PL map, collected at 3 µm intervals [34].

5.2. PL Mapping Provides Clues to Unusual Origin Story The “Matryoshka” diamond is a 0.62 ct greenish rough diamond with a freely moving diamond trapped inside [35]. The exterior and interior diamonds both showed visual evidence of radiation stains and these observations were confirmed by PL mapping using 633 nm excitation and plotting the GR1 feature [V0] at 741.2 nm. Figure 10A shows a visual camera mosaic generated by the mapping instrument, indicating the internal diamond in focus and the external diamond out of focus. The PL maps were collected in confocal mode to best distinguish the PL spectra from the inner and outer diamond. Figure 10B shows the PL map of the GR1 defect. It shows generally a higher intensity of GR1 in the outer Figure 9. (Left) DiamondViewdiamond, fluorescence yet shows image showedsome high interesting GR1 emission green fluorescence detected aroundin some the portions cavities onof thethe crown inner dia- facet of a 0.70 ct E-color diamond.mond The (indicated green fluorescent by red arrow haloes werein Figure created 10). by This radioactive confirms fluids that (Center radioactive). Subsequent fluids PL pene- mapping with 532 nm excitationtrated showed inside that the high cavity amounts and came of radiation into physical brought about contact a number with the oflocalized internal changes diamond in thecrystal. diamond surrounding the cavities.This type (Right of )analysis A line scan would (indicated have bybeen a black far more line in difficult the central to image)achieve shows with thestandard normalized PL spec- peak intensity for the GR1 fromtroscopy. the PL map, collected at 3 µm intervals [34].

Figure 10. The 0.62 ct greenish rough diamond was termed the “Matryoshka” diamond because of the free-moving internal Figure 10. The 0.62 ct greenish rough diamond was termed the “Matryoshka” diamond because of the free-moving internal diamond. (A) Camera mosaic generated by the mapping instrument shows the internal diamond in focus and the external diamond. (A) Camera mosaic generated by the mapping instrument shows the internal diamond in focus and the external diamond out of focus; the confocal PL mapping was collected at the corresponding focal plane. (B) The PL map of the diamondnormalized out GR1 of focus;defect theintensity confocal at 741.2 PL mapping nm (normalized was collected usingat the the diamond corresponding Raman focalpeak plane.at 691 nm) (B) Thewas PLcollected map of with the normalized633 nm excitation GR1 defect at liquid intensity nitrogen at 741.2 temperature. nm (normalized For this using map, the~34,000 diamond individual Raman spectra peak at were 691 nm)collected was collectedwith a 25 with µm 633pixel nm size excitation over a collection at liquid nitrogentime of 7½ temperature. min. The red For arrow this map, in both ~34,000 figures individual shows an spectra area of were high collected GR1 intensity with a 25on µthem 1 pixelsurface size of over the ainternal collection diamond. time of For 7 2 min. this Thediamond, red arrow we did in both not ﬁguresuse graphite shows paint an area (e.g., of highFigure GR1 2) intensityas we were on theconcerned surface ofabout the internalthe paint diamond. potentially For entering this diamond, the internal we did cavity not useand graphiteinstead fixed paint graphite (e.g., Figure tape2 )to as the we bott wereom concerned of the sample about while the paintimaging. potentially entering the internal cavity and instead ﬁxed graphite tape to the bottom of the sample while imaging.

6. Inclusion Analysis within Natural Diamonds Diamonds, andand theirtheir associatedassociated inclusions, inclusions, can can provide provide fascinating fascinating new new discoveries discoveries of newof new minerals minerals [36 ].[36] Diamond. Diamond inclusions inclusions can providecan provide windows windows into the into Earth’s the E lithosphericarth’s litho- andspheric sub-lithospheric and sub-lithospheric mantle, providing mantle, providing us with insight us with into theinsight processes into the and processes compositions and ofcompositions what is below of what us. With is below the proper us. With equipment the proper and equipment procedures, and these procedures, diamond inclusionsthese dia- canmond be inclusions analyzed andcan identiﬁed.be analyzed and identified. Raman spectroscopy is widely used in the identification of diamond inclusions; Ra- man spectroscopy is generally the best method to identify non-surface-reaching inclusions and has been used to discover a number of different inclusions, such as inclusions formed in superdeep environments [37] or inclusions with different paragenetic origins [38]. Minerals 2021, 11, 177 11 of 31

Minerals 2021, 11, x FOR PEER REVIEW 12 of 34

Raman spectroscopy is widely used in the identification of diamond inclusions; Raman spectroscopy is generally the best method to identify non-surface-reaching inclusions and has been used to discover a number of different inclusions, such as inclusions formed in Diamond inclusions can exhibit many different features that make them difficult to superdeep environments [37] or inclusions with different paragenetic origins [38]. identify. They can contain several different minerals, can come in various sizes (from a Diamond inclusions can exhibit many different features that make them difficult to few microns to about a millimeter), and they can contain coatings that make them practi- identify. They can contain several different minerals, can come in various sizes (from a few cally impossible to identify. The proper instrumentation and data collection method is microns to about a millimeter), and they can contain coatings that make them practically especially crucial for identifying diamond inclusions. impossible to identify. The proper instrumentation and data collection method is especially Raman mapping is an efficient method for identifying very small inclusions or inclu- crucial for identifying diamond inclusions. sions with multiple mineral assemblages. Figure 11 shows an inclusion in a type IIa pink Raman mapping is an efficient method for identifying very small inclusions or inclu- diamond with a graphitized rosette stress fracture around it, with the inclusions mainly sions with multiple mineral assemblages. Figure 11 shows an inclusion in a type IIa pink beingdiamond a semi with-transparent a graphitized color. rosette With stress Raman fracture mapping, around this it, inclusion with the inclusionswas identifi mainlyed as mineralbeing a breyite semi-transparent (CaSiO3) along color. with With a pronounced Raman mapping, graphitic this halo. inclusion Since inclusions was identified such as breyite have yet to be found in known lithospheric diamonds, it has been inferred that mineral breyite (CaSiO3) along with a pronounced graphitic halo. Since inclusions such as thesebreyite diamonds have yet tolikely be found have ina knownsub-lithospheric lithospheric origin diamonds, at depths it has of beenaround inferred 360 to that 750 these km [13]diamonds. likely have a sub-lithospheric origin at depths of around 360 to 750 km [13].

Figure 11. Top left: Photomicrograph of the breyite inclusion located within a type IIa pink diamond; the red box shows Figure 11. Top left: Photomicrograph of the breyite inclusion located within a type IIa pink diamond; the red box shows the the area being mapped. Top center: False color map showing the spatial distribution of graphite around the breyite inclusion.area beingTop right: mapped. False Top color center: map Falseshowing color the map spatial showing distribution the spatial of distributionthe 640–680 ofcm graphite−1 peak found around in the the breyite breyite inclusion. spectra. −1 Bottom:Top right: Ra Falseman colorspectrum map showing showing the the identification spatial distribution of the ofbreyite the 640–680 inclusion cm withinpeak the found image in. the breyite spectra. Bottom: Raman spectrum showing the identification of the breyite inclusion within the image. Photoluminescence mapping of another diamond showed chromium-related peaks Photoluminescence mapping of another diamond showed chromium-related peaks at 693 and 694 nm, which were present at distinct positions. The specific locations of the at 693 and 694 nm, which were present at distinct positions. The specific locations of chromium emission correlated to inclusions within the diamond (Figure 12). Raman spec- the chromium emission correlated to inclusions within the diamond (Figure 12). Raman troscopy identified numerous corundum inclusions ranging from 0.18 (Figure 12) to 0.07 spectroscopy identified numerous corundum inclusions ranging from 0.18 (Figure 12) to mm in size. By viewing the inclusions with a diffuser plate, a pale pink color was observed 0.07 mm in size. By viewing the inclusions with a diffuser plate, a pale pink color was through some of the inclusions. Compiling all this evidence, it was clear that the inclusions observed through some of the inclusions. Compiling all this evidence, it was clear that the were corundum, either ruby or pink sapphire, in this gem-quality diamond. To our inclusions were corundum, either ruby or pink sapphire, in this gem-quality diamond. To knowledge, these inclusions are the second occurrence of chromium-rich corundum (ruby or pink sapphire) found in a natural gem diamond [39]. Minerals 2021, 11, x FOR PEER REVIEW 13 of 34

Minerals 2021, 11, 177 12 of 31 Whether gemologists are searching for rare corundum inclusions in gem quality diamonds or finding inclusions that originated from the sub-lithosphere, using a Raman imagour knowledge,ing microscope these can inclusions be a great are tool the for second diamond occurrence inclusion of identification. chromium-rich corundum (ruby or pink sapphire) found in a natural gem diamond [39].

FigureFigure 12. 12. ((AA)) A A 0.13 0.13 ct ct diamond diamond show shownn to to have have corundum corundum inclusions. inclusions. ( (BB)) Photomicrograph Photomicrograph of of the the Table Table 693 693 nm nm chromium chromium peaks (532 nm excitation).excitation). ((CC)) PhotoluminescencePhotoluminescence spectrum spectrum showing showing the the detection detection of of chromium chromium with with 532 532 nm nm laser laser excitation. excitation.(D) Raman (D) Raman spectra spectra conﬁrming confirming the corundum the corundum identiﬁcation. identification The. blue The traceblue showstrace shows the submitted the submitted diamond diamond inclusion, inclusion, while while the red trace is a known corundum reference. Spectra vertically offset for clarity [39]. the red trace is a known corundum reference. Spectra vertically offset for clarity [39].

Whether gemologists are searching for rare corundum inclusions in gem quality diamonds or ﬁnding inclusions that originated from the sub-lithosphere, using a Raman imaging microscope can be a great tool for diamond inclusion identiﬁcation. Minerals 2021, 11, 177 13 of 31

7. PL Mapping of Laboratory-Grown Diamonds Minerals 2021, 11, x FOR PEER REVIEW 7.1. HPHT-Grown Diamonds 15 of 36 In the 1990s, DeBeers developed the DiamondView ﬂuorescence imaging microscope (using ultra-shortwave excitation of ~225 nm) to distinguish HPHT-grown diamonds from their natural counterparts [40]. The basis for the DiamondView was that the growth morphologydistinct growth for HPHTstructure synthetics is vividly is quite recor differentded within from the naturally fluorescence grown as diamonds. the different The distinctgrowth growthsectors structureallowed for is vividlyvarying recorded incorporation within of the the ﬂuorescence defects (Figure as the 13A). different This growth differ- sectorsence in allowedimpurity for concentration varying incorporation also revealed of thethe defectsoutlines (Figure of the growth13A). This sectors. difference This un- in impurityderlying mechanism concentration has also also revealed proven useful the outlines to study of HPHT-grown the growth sectors. diamonds This underlyingby PL map- mechanismping (Figure has13B,C), also which proven allows useful quantitative to study HPHT-growncomparison of diamondsthe detected by defects PL mapping and the (Figureability to 13 studyB,C), whichthe trends allows between quantitative them (F comparisonigure 13D). Thus, of the far detected more detailed defects analyses and the abilityof the interrelationships to study the trends of between various themdefects (Figure are now 13D). possible. Thus, far more detailed analyses of the interrelationships of various defects are now possible.

FigureFigure 13.13.( A(A)) A A 0.46 0.46 ct Fancyct Fancy Vivid Vivid yellow yellow HPHT-grown HPHT-grown diamond diamond is illuminated is illuminated by the DiamondViewby the DiamondView microscope, microscope, showing theshowing various the intensities various intensities and colors and of fluorescence colors of fluorescence across the growthacross the sectors. growth (B) sectors. The PL map(B) The for PL the map neutral for nitrogen-vacancythe neutral nitro- (NVgen-vacancy0) center—ZPL (NV0) center—ZPL = 575 nm; (C =) 575 PL mapnm; ( forC) PL the map negatively for the chargednegatively nitrogen-vacancy charged nitrogen-vacancy (NV−) center—ZPL (NV-) center—ZPL = 637 nm; = 0 - (637D) Plotnm; of(D the) Plot values of the for values NV0 forvs. NV NV− vs.across NV across the region the region indicated indicated by the by black the boxblack in box (B). in It ( showsB). It shows a consistent a consistent ratio betweenratio between these these two centerstwo centers across across the growththe growth sectors, sectors, thus th providingus providing more more information information than than can can be be determined determined from from a a DiamondView image alone. DiamondView image alone. 7.2. CVD-grown CVD-Grown Diamonds Diamonds PL mapping has also proven usefuluseful forfor syntheticsynthetic diamondsdiamonds growngrown byby thethe chemicalchemical vapor deposition (CVD) method; it has been helpful not only forfor identifyingidentifying CVD-grownCVD-grown diamonds [[41],41], but also to perform more fundamentalfundamental research onon the growth process.process. For example, with a DiamondView microscope,microscope, wewe cancan oftenoften andand easilyeasily seesee a growth interface that indicates indicates a a stop/start stop/start growth growth event event in inCVD-grown CVD-grown diamonds. diamonds. The The fluorescence fluorescence at that at thatgrowth growth interface interface is often is distinct often distinct and different and different from the from fluorescence the fluorescence seen within seen the within bulk theof the bulk CVD of thegrowth. CVD PL growth. mapping PL mappinganalyzed analyzedin greater in detail greater the detail defects the that defects are detected that are detectedwithin that within interface that and interface how those and how compare those with compare the bulk with of the the bulk CVD of growth the CVD layer growth (Fig- layerure 14). (Figure These 14 maps). These show maps a number show aof number differences of differences between the between layers theand layers an increased and an increasedconcentration concentration of vacancy-related of vacancy-related defects at the defects interfaces at the that interfaces decreases that with decreases growth withtime growthwithin each time layer. within each layer. Minerals 2021, 11, 177 14 of 31 Minerals 2021, 11, x FOR PEER REVIEW 15 of 34

Figure 14. (Top) A thin laser-cut slice from a CVD-grown diamond composed of five different Figure 14. (Top) A thin laser-cut slice from a CVD-grown diamond composed of five different layers layers from five consecutive growth runs using a natural diamond substrate. The sample is shown from five consecutive growth runs using a natural diamond substrate. The sample is shown with with 2 types of illumination: visible and DiamondView (fluorescence) illumination, showing the 2distinct types of growth illumination: interfaces. visible (Bottom and DiamondView) Spectral data (fluorescence) exported from illumination, the PL map showing(along the the red distinct line growthshown interfaces. in top image) (Bottom show) Spectral the change data in exported vacancy from-related the defects PL map (NV (along0 at the575 red nm, line NV shown- at 637 in nm, top − image)and the show negative the change silicon in-vacancy vacancy-related defect at defects 737 nm) (NV due0 at to 575 the nm, growth NV interruptions.at 637 nm, and the negative silicon-vacancy defect at 737 nm) due to the growth interruptions. 8. Blue Sapphire: Geographic Origin Determination 8. Blue Sapphire: Geographic Origin Determination Over the last few decades, geographic origin determination has become a major mo- Over the last few decades, geographic origin determination has become a major tivating force in gemological research. As the gem and jewelry industry has migrated to- motivating force in gemological research. As the gem and jewelry industry has migrated wards using a gem’s geographic origin as a factor in determining value, they have relied towards using a gem’s geographic origin as a factor in determining value, they have on gemological laboratories to determine origin based on gemological, physical, and relied on gemological laboratories to determine origin based on gemological, physical, and chemical properties. However, as the number of economically important gem deposits chemical properties. However, as the number of economically important gem deposits has has increased in modern times, so has the overlap in properties of stones from geograph- increased in modern times, so has the overlap in properties of stones from geographically ically distinct deposits. distinct deposits. The problem can be particularly acute in the case of metamorphic blue sapphires such The problem can be particularly acute in the case of metamorphic blue sapphires suchas those as those found found in Sri in Lanka, Sri Lanka, Myanmar, Myanmar, Indian Indian Kashmir, Kashmir, and andMadagascar Madagascar [42]. [ There42]. There exists existssignifican significantt overlap overlap in the in trace the traceelement element profiles profiles for sapphires for sapphires from fromthese these deposits deposits;; there- therefore,fore, the thegemologist gemologist must must rely rely largely largely on on microscopic microscopic observations observations in in order order to to conclude a astone’s geographic origin. TheThe mostmost commoncommon inclusionsinclusions observed observed in in the the microscope microscope are are fine particles of rutile that are often crystallographically aligned with the corundum host Minerals 2021, 11, 177 15 of 31

Minerals 2021, 11, x FOR PEER REVIEW 16 of 34

ﬁne particles of rutile that are often crystallographically aligned with the corundum host and called “silk” by the gemologist. Studying differences in the pattern of his “silk” and comparingand called against “silk” by references the gemologist. stones Studying with known differences provenance in the pattern can often of his help “silk” to and determine a comparing against references stones with known provenance can often help to determine stone’s geographic origin. a stone’s geographic origin. AnotherAnother feature feature thatthat can can aid aid in in this this endeavor endeavor is the is presence/absence the presence/absence of color ofzoning. color zoning. SapphiresSapphires from from Madagascar Madagascar or Sri Lanka Lanka may may show show dramatic dramatic color color zoning zoning with alternat- with alternating blueing and blue colorless and colorless zones zones constrained constrained to follow follow specific speciﬁc crystallographic crystallographic planes planesand can and can tracktrack the the crystal crystal growth growth history of of the the gems. gems. The The blue blue color color is derived is derived from absorption from absorption of of redred light light through through an an intervalenceintervalence charge charge transfer transfer between between Fe2+ Feand2+ Tiand4+ sitting Ti4+ onsitting adjacent on adjacent octahedraloctahedral sites sites in in thethe corundumcorundum structure structure [43] [43; therefore,]; therefore, the oscillatory the oscillatory zoning zoningindicates indicates

differencesdifferences in in the the chemicalchemical environment environment during during growth growth involving involving not only not iron only (Fe iron) and (Fe) and titanium (Ti) but also potentially magnesium (Mg) and silicon (Si) [44]. In Madagascar and titanium (Ti) but also potentially magnesium (Mg) and silicon (Si) [44]. In Madagascar and Sri Lankan sapphires, the color zoning usually occurs with very sharp boundaries be- Sritween Lankan blue sapphires, and colorless the zones color and zoning generally usually follows occurs a hexagonal with very pattern sharp when boundaries looking between bluedown and the colorless c-axis (Figure zones 15 and). On generally the other followshand, Burmese a hexagonal sapphires pattern often have when very looking ho- down themogeneous c-axis (Figure blue 15 coloration,). On the or other at most hand,, they Burmese may show sapphires color zoning often with have diffuse very bound- homogeneous bluearies coloration, between blue or at and most, colorless they zones may, showwhere the color blue zoning gradually with transitions diffuse boundariesinto colorless between blue(Figure and colorless16). In this zones, case, the where color thezoning blue preserves gradually information transitions about into the colorless unique geolog- (Figure 16). In thisical case, forces the that color created zoning these preserves gems and can information be used to help about determine the unique a stone’s geological geographic forces that createdorigin. these gems and can be used to help determine a stone’s geographic origin.

Minerals 2021, 11, x FOR PEER REVIEW 18 of 36 Figure 15. Photo of a Madagascar blue sapphire wafer showing hexagonal blue color zoning with sharp boundaries be- Figure 15. Photo of a Madagascar blue sapphire wafer showing hexagonal blue color zoning with sharp boundaries between tween blue and colorless zones (left). On the (right) is the Cr distribution map of the same sapphire created by PL map- blue andping. colorless zones (left). On the (right) is the Cr distribution map of the same sapphire created by PL mapping.

Figure 16. PhotoFigure of a16. Burmese Photo of a blueBurmese sapphire blue sapphire wafer wafer showing showing diffusediffuse color color zoning zoning (left). (Onleft the). right On is the the ( Crright distribution) is the Cr distribution map of the same sapphire created by PL mapping. map of the same sapphire created by PL mapping. Using color zoning to provide information on geographic origin is essentially using information about the spatial distribution of trace elements, specifically Fe and Ti in the case of sapphires. Hypothetically, having the ability to map out distributions of any trace elements in corundum may also provide additional information that could provide a unique fingerprint leading back to a stone’s geographic origin. Trace element mapping with laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) has been performed on gem corundum before (e.g., [45]), but the LA-ICP-MS mapping method is time-intensive and destructive. Another option is to use PL mapping to look for variations in the intensity of the Cr3+ luminescence peaks centered at 692.9 and 694.3 nm (see, for example, Figure 12C) in order to map out variation in chromium (Cr) concentrations within a single crystal. PL mapping has been used before to delineate the crystal growth history of rubies and blue sapphires from Yogo Gulch, Montana [15,46]. Of particular note is that the distribution of Cr in Yogo sapphires can reveal complicated growth patterns that would not be visible in these sapphires that otherwise show homogeneous distribution of color and other trace elements. For whatever reason, Cr often seems to become incorporated very differently in corundum than the other trace elements (e.g., [47]). PL maps were produced for the sharply blue-zoned Madagascar sapphire in Figure 15 and the diffuse-zoned Burmese sapphire in Figure 16. In both cases, the Cr-distribution identified by PL mapping correlates well with the observed color zoning. In some cases, however, PL mapping can shed light on crystal growth patterns that do not show up through observation of color zoning (Figure 17). Minerals 2021, 11, 177 16 of 31

Using color zoning to provide information on geographic origin is essentially using information about the spatial distribution of trace elements, specifically Fe and Ti in the case of sapphires. Hypothetically, having the ability to map out distributions of any trace elements in corundum may also provide additional information that could provide a unique fingerprint leading back to a stone’s geographic origin. Trace element mapping with laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) has been performed on gem corundum before (e.g., [45]), but the LA-ICP-MS mapping method is time-intensive and destructive. Another option is to use PL mapping to look for variations in the intensity of the Cr3+ luminescence peaks centered at 692.9 and 694.3 nm (see, for example, Figure 12C) in order to map out variation in chromium (Cr) concentrations within a single crystal. PL mapping has been used before to delineate the crystal growth history of rubies and blue sapphires from Yogo Gulch, Montana [15,46]. Of particular note is that the distribution of Cr in Yogo sapphires can reveal complicated growth patterns that would not be visible in these sapphires that otherwise show homogeneous distribution of color and other trace elements. For whatever reason, Cr often seems to become incorporated very differently in corundum than the other trace elements (e.g., [47]). PL maps were produced for the sharply blue-zoned Madagascar sapphire in Figure 15 and the diffuse- zoned Burmese sapphire in Figure 16. In both cases, the Cr-distribution identified by PL mapping correlates well with the observed color zoning. In some cases, however, PL Minerals 2021, 11, x FOR PEER REVIEW 18 of 34 mapping can shed light on crystal growth patterns that do not show up through observation of color zoning (Figure 17).

FigureFigure 17. Photo17. Photo of of a Burmesea Burmese blueblue sapphire waf waferer with with homogeneous homogeneous blue bluecoloration coloration (left). On (left the). (right On the) is the (right Cr )dis- is the Cr tribution map of the same sapphire created by PL mapping. distribution map of the same sapphire created by PL mapping. Sapphires from Madagascar, Sri Lanka, and Burma were included in this preliminary Sapphires from Madagascar, Sri Lanka,3+ and Burma were included in this preliminary study to determine if PL mapping of Cr lum3+inescence can aid in geographic origin de- studytermination to determine of metamorphic if PL mapping blue sapphires. of Cr Theluminescence sapphires studied can aidhere in were geographic all cut as origin determinationwafers with the of corundum metamorphic c-axis blueperpendicular sapphires. to the The polished sapphires faces studied in order hereto keep were the all cut asorientation wafers with consistent the corundum for this initial c-axis work. perpendicular Several representative to the polished examples faces of in Cr order-distri- to keep thebution orientation maps for consistent each locale forare thisshown initial in Figure work.s 18 Several–20. In general, representative the Cr maps examples seem to of Cr- distributionfollow the same maps general for each trends locale seen are with shown color zoning in Figures in these 18– 20stones. In, general,with sharp, the dra- Cr maps seemmatic to Cr follow-zoning the being same common general in Sri trends Lankan seen and with Madagascar color zoning sapphires in and these generally stones, with sharp,subtler dramatic Cr-zoning Cr-zoning being more being common common in Burmese in Sri Lankan sapphires. and Observation Madagascar of sapphiresangular, and generallyhexagonal subtler banding Cr-zoning is more common being more in Madagascar common and in Burmese Sri Lankan sapphires. sapphires. ObservationNotably, of angular,the cores hexagonal of several bandingof these sapphires is more seem common to show inMadagascar complex initial and growth Sri Lankan patterns sapphires. involving growth along more than one crystal form involving more than one set of planes Notably, the cores of several of these sapphires seem to show complex initial growth (Figures 18B,D and 19B). However, Burmese sapphires are more likely to have homoge- patterns involving growth along more than one crystal form involving more than one set neous distributions of Cr (Figure 20A) or bands of Cr enrichment with diffuse boundaries of(Figure planes 20B (Figures,C). Less 18 B,Dcommonly and 19, BBurmese). However, sapphires Burmese may show sapphires Cr-enriched are more bands likely with to have homogeneoussharper boundaries distributions (Figure 20D of). Cr (Figure 20A) or bands of Cr enrichment with diffuse boundariesHowever, (Figure these 20 generalizationsB,C). Less commonly, do not cover Burmese the full sapphires range of possibilities may show forCr-enriched Cr bandsmaps with seen sharper for these boundaries deposits. For (Figure instance, 20D). some Burmese sapphires will occasionally show distinct angular banding in their Cr distribution maps. The Cr maps for the Burmese sapphires in Figures 17 and 20D,F appear to be nearly indistinguishable from those shown for Madagascar or Sri Lankan sapphires in Figures 18A,C and 19E. Additionally, some Sri Lankan and Madagascar sapphires will show only diffuse zones of Cr enrichment, as in Figures 18F and 19F, which may not be distinguishable from the typical patterns seen for Burmese sapphires. However, the more complex growth patterns seen in some Sri Lankan and Madagascar sapphires which deviate from the strict angular banding with hexagonal geometry (Figures 18B,D,E and 19A–D) have never been seen in Burmese sapphires. Ob- servation of this type of growth through PL mapping could be useful to exclude Burma as a possible origin for some challenging cases of origin determination. Further work is needed to more clearly delineate how and when PL mapping can be useful for geographic origin determination of metamorphic blue sapphires. Future research should also focus on the potential use of PL mapping for geographic origin determination of basalt-related blue sapphires as well as rubies. Minerals 2021, 11, 177 17 of 31 Minerals 2021, 11, x FOR PEER REVIEW 19 of 34 Minerals 2021, 11, x FOR PEER REVIEW 19 of 34

FigureFigure 18. 18. RepresentativeRepresentative PL PL maps maps showing showing Cr Cr distribution distribution in in Madagascar Madagascar sapphires sapphires demarcating demarcating ( (AA––EE)) sharp, sharp, oscillatory oscillatory Figure 18. Representative PL maps showing Cr distribution in Madagascar sapphires demarcating (A–E) sharp, oscillatory growthgrowth patterns, patterns, often often arranged arranged in hexagonal patterns, as well as ( F) diffuse variations in Cr concentrations.concentrations. growth patterns, often arranged in hexagonal patterns, as well as (F) diffuse variations in Cr concentrations.

Figure 19. Representative PL maps showing Cr distribution in Sri Lankan sapphires demonstrating (A–E) sharp, oscilla- FigureFigure 19. 19. RepresentativRepresentativee PL maps showing Cr distributiondistribution inin SriSri LankanLankan sapphiressapphires demonstrating demonstrating (A (A–E–)E sharp,) sharp, oscillatory oscillatory growth patterns, often arranged in hexagonal patterns, as well as (F) diffuse variations in Cr concentrations. Note, torygrowth growth patterns, patterns, often often arranged arranged in in hexagonal hexagonal patterns, patterns, as aswell well asas (F) diffuse diffuse variations variations in in Cr Cr concentrations. concentrations. Note, Note, (A) reproduced from [48]. ((AA)) reproduced reproduced from from [48] [48].. MineralsMinerals2021, 202111, 177, 11, x FOR PEER REVIEW 21 of 36 18 of 31

Figure 20. Representative PL maps showing Cr distribution in Burmese sapphires showing (A) the near absence of Figure 20. Representative PL maps showing Cr distribution in Burmese sapphires showing (A) the near absence of spa- spatial variations in Cr concentrations, (B,C) growth zoning with diffuse boundaries, and (D–F) growth zoning with sharp tial variations in Cr concentrations, (B,C) growth zoning with diffuse boundaries, and (D–F) growth zoning with sharp boundariesboundaries arranged arranged in hexagonal in hexagonal patterns. patterns. However, these generalizations do not cover the full range of possibilities for Cr maps seen9. Raman for these Mapping deposits. of Tourmaline For instance, for Species some Classification Burmese sapphires will occasionally show distinctIn angulargemology, banding LA-ICP-MS in their has been Cr distributionused to analyze maps. both major The Crand maps trace elements for the Burmeseof sapphiresgem quality in Figures tourmaline 17 and for 20geographicD,F appear origin to bedetermination nearly indistinguishable [49,50] and species from classifi- those shown forcation Madagascar [51]. The orlocalized Sri Lankan analytical sapphires techniqu ine produces Figures 18 a A,Crepresentative and 19E. composition Additionally, of some the gemstone, and thus, species identification, if the sample is chemically homogeneous. Sri Lankan and Madagascar sapphires will show only diffuse zones of Cr enrichment, However, many tourmaline gems are zoned and information may be missed if the tour- asmaline in Figures is chemically 18F and hetero19F, whichgeneous. may Raman not bemapping distinguishable can be used from as thean alternative typical patterns seenmethod for Burmese to chemically sapphires. characterize However, the sample the moreat a small complex scale and growth resolve patterns this issue. seen in some Sri LankanTourmalines and Madagascar from Anjanabonoina, sapphires Madagascar which deviate are known from for the their strict complex angular color banding withzoning. hexagonal Chemical geometry analyses (Figuressuggest that 18B,D,E the zones and are 19 A–D)composed have of never elbaite been and liddicoatite seen in Burmese sapphires.[52], similar Observation to the pear-shaped of this tourmaline type of growth sample throughshown in PLFigure mapping 21. A traverse could of be LA- useful to excludeICP-MS Burma analyses as were a possible performed origin across for the some length challenging of the sample. cases Using of origin these determination.data, the Furthertwo laser work ablation is needed areas on to the more thickest clearly black delineate band of the how sample and in when Figure PL 21 were mapping classi- can be usefulfied as for elbaite geographic [51], the origin sodium-, determination lithium-, aluminum-rich of metamorphic tourmaline blue sapphires. species (e.g., Future [53]). research shouldThe remaining also focus areas on thewere potential classified use as liddicoatite, of PL mapping the calcium-, for geographic lithium-, originaluminum-rich determination oftourmaline basalt-related species. blue Obviously, sapphires one as will well not as rubies.always have the luxury to place several laser ablation spots on the table of a fine gemstone. However, Raman mapping offers a new 9.tool Raman to identify Mapping tourmaline of Tourmaline species and for toSpecies study the Classification chemical zoning of a gem tourmaline in an accurate, non-destructive, and acceptable way. In gemology, LA-ICP-MS has been used to analyze both major and trace elements of gem quality tourmaline for geographic origin determination [49,50] and species classification [51]. The localized analytical technique produces a representative composition of the gemstone, and thus, species identification, if the sample is chemically homogeneous. However, many tourmaline gems are zoned and information may be missed if the tourmaline is chemically heterogeneous. Raman mapping can be used as an alternative method to chemically characterize the sample at a small scale and resolve this issue. Tourmalines from Anjanabonoina, Madagascar are known for their complex color zoning. Chemical analyses suggest that the zones are composed of elbaite and liddicoatite [52], similar to the pear-shaped tourmaline sample shown in Figure 21. A traverse of LA-ICP-MS analyses were performed across the length of the sample. Using these data, the two laser ablation areas on the thickest black band of the sample in Figure 21 were classified as elbaite [51], the sodium-, lithium-, aluminum-rich tourmaline species (e.g., [53]). The remaining areas were classified as liddicoatite, the calcium-, lithium-, aluminum-rich Minerals 2021, 11, 177 19 of 31

Minerals 2021, 11, x FOR PEER REVIEWtourmaline species. Obviously, one will not always have the luxury to place several21 of 34 laser

Minerals 2021, 11, x FOR PEER REVIEW ablation spots on the table of a ﬁne gemstone. However, Raman22 of 36 mapping offers a new tool to identify tourmaline species and to study the chemical zoning of a gem tourmaline in an accurate, non-destructive, and acceptable way.

Figure 21. A pear-shaped step-cut gem quality tourmaline stone from Anjanabonoina, Madagas- Figure 21. A pear-shaped step-cut gem quality tourmaline stone from Anjanabonoina, Madagascar. car. AnFigure area was 21. analyzed A pear by-shaped the Raman step mapping-cut gem technique quality (blue tourmaline rectangle; color stone scale fromshown Anjanabonoina,in Madagas- FigureAn area23). Laser was ablation analyzed (LA-ICP-MS) by craters the Raman are visible mappingas dots across techniquethe table of the (bluesample. rectangle; color scale shown in Chemicalcar. dataAn obtainedarea was by analyzedLA-ICP-MS byclassify the theRam thickestan mapping black band technique as elbaite, with (blue the remain-rectangle; color scale shown in ingFigure sampleFigure 23).classified 23). Laser L aseras liddicoatite. ablation Sample (LA (LA-ICP-MS)- ICPcourtesy-MS) of cratersBarbara craters L. are Dutrow. visible are visible as dots as across dots the across table the of the table sample. of the sample. ChemicalChemical data data obtained obtained by by LA-ICP-MSLA-ICP-MS classif classifyy the the thickest thickest black black band band as elbaite as elbaite,, with with the remain- the remaining ingBy using sample a ratio classified of the integrated as liddicoatite. green area Sample 1 to the courtesy red area of2 inBarbara Figure 22,L. Dutrow.a third parametersample was classiﬁed developed as and liddicoatite. used for mapping. Sample Elbaitic courtesy tourmaline of Barbara has a much L. stronger Dutrow. band at around 840 cm−1, and a much larger ratio of the area 1 to 2 compared to liddicoatite [54]. TheBy ratioBy using ofusing these two aa ratio areas ofdifferentiates of the the integrated integrated the two speciesgreen green clearly area in1 area theto themap; 1 tored the the areasarea red 2 in areaFigure 2 in22, Figurea third 22, a that areparameter composed was of mostly developed elbaite appear and usedas red, for while mapping. the areas thatElbaitic are composed tourmaline of has a much stronger mostlythird liddicoatite parameter appear was as blue. developed The estimated and ratio used that forseparates mapping. elbaite and Elbaitic lid- tourmaline has a much band at around 840 cm−1, and a much− larger ratio of the area 1 to 2 compared to liddicoatite dicoatitestronger is around band 1.5. at around 840 cm 1, and a much larger ratio of the area 1 to 2 compared to [54]. The ratio of these two areas differentiates the two species clearly in the map; the areas liddicoatite [54]. The ratio of these two areas differentiates the two species clearly in the that are composed of mostly elbaite appear as red, while the areas that are composed of map;mostly the areas liddicoatite that are appear composed as blue. of The mostly estimated elbaite ratio appear that as separates red, while elbaite the and areas lid- that are composeddicoatite ofis around mostly 1.5. liddicoatite appear as blue. The estimated ratio that separates elbaite and liddicoatite is around 1.5.

FigureFigure 22. Raman 22. Raman spectra spectra of twoof two pixels pixels in in the the RamanRaman map. The The bottom bottom trace trace represents represents a pixel a pixel (pixel (pixel 1 in Figure 1 in Figure 23) 23) classified as elbaite while the top trace represents a pixel (pixel 2 in Figure 23) classified as liddicoatite. The ratio of the classified as elbaite while the top trace represents a pixel (pixel 2 in Figure 23) classified as liddicoatite. The ratio of the integrated green area 1 to the red area 2 can be plotted as the third parameter in the map to differentiate these two species. This allows higher resolution species identification. Minerals 2021, 11, 177 20 of 31

One portion of this sample had a high-resolution Raman map with a 455 nm polarized Minerals 2021, 11, x FOR PEER REVIEW 24 of 36 laser at room temperature with 0.5 s exposure time, and 10 µm pixel size (Figures 21 and 23). The total time used to generate this map was 16 h and 24 min. A map with lower resolution in Figure 24 was also generated to compare to Figure 23. The total time used to generate this map is 48 min with a larger 50 µm pixel size. Therefore, it is feasible to map a

Minerals 2021, 11, x FOR PEER REVIEWproduction stone at lower resolution and shorter time. 24 of 36 The ability to accurately measure the overall composition of chemically zoned tourmaline gemstones using a non-destructive method could be a useful application in a gemological laboratory.

Figure 23. High-resolutionFigure 23. High-resolution Raman map of a Raman portion map of the of a tourmalin portion ofe theshown tourmaline in Figure shown 21. Colo in Figurer bar represents21. Color bar the represents ratio the ratio of of area 1 to area 2 (see FigureFigure 22), 23. High-resolutionwhich varies across Raman the map anal of ayzed portion portion. of the tourmalin Pixel sizee shown is 10 µmin Figure and it21. required Color bar 16 represents h and 24 the ratio area 1 toof area area 2 1 (see to area Figure 2 (see 22 Figure), which 22), which varies varies across across the analyzedthe analyzed portion. portion. Pixel Pixel size is is 10 10 µmµm and and it required it required 16 h 16and h 24 and 24 min min to generate this map. to generatemin thisto generate map. this map.

Figure 24. A low-resolution Raman map of the similar portion of the tourmaline sample shown in Figure 21. Pixel size is 50 µm and it took 48 min to generate this map.

Figure 24. A low-resolution Raman10. Raman map of Mapping the similar for portionSeparation of theof Jadeite tourmaline and Omphacite sample shown in Figure 21. Pixel size is Figure 24. A low-resolution Raman map of the similar portion of the tourmaline sample shown in 50 µm and it took 48 min to generate10.1. Chemical this map. Distinction between Jadeite and Omphacite Figure 21. Pixel size is 50 µm and it took 48 min to generate this map.

10. Raman Mapping for Separation of Jadeite and Omphacite 10.1. Chemical Distinction between Jadeite and Omphacite Minerals 2021, 11, 177 21 of 31

10. Raman Mapping for Separation of Jadeite and Omphacite 10.1. Chemical Distinction between Jadeite and Omphacite Jadeite and omphacite are related members of the pyroxene mineral group with monoclinic crystal symmetry. As a result, most gemological tests give similar results for both minerals (e.g., refractive index (R.I.) = 1.65–1.69 for jadeite, 1.66–1.72 for omphacite; specific gravity (S.G.) = 3.3 for jadeite, 3.34 for omphacite). The common thinking for identification has long been that if a stone is a member of the jadeite group, then the color can adequately serve as a separation tool. In the jewelry industry, very dark green to black material, as well as bluish “Guatemalan jade,” have been considered omphacite, whereas light to medium-colored violet, white, green, and mottled stones were identified as jadeite. This color-based separation of jadeite pyroxenes seemed to work well until the recent revelation that some gem-quality medium green stones that visually resembled traditional jadeite jade were, in fact, composed completely of omphacite [55–57]. To mineralogists, the difference between jadeite and omphacite is caused by the substitution of particular chemical elements in the atomic structure of the minerals. The 3+ chemical formula for jadeite is Na(Al,Fe )Si2O6, and for omphacite, it is (Ca,Na)(Mg, Al)Si2O6. In mineral formulas, the elements in parentheses can substitute for each other because they can fit into the same lattice positions. From examining these formulas, the only significant differences between these two related silicate minerals are the amount of sodium (Na) and calcium (Ca) in the first formula position and the amounts of aluminum (Al), magnesium (Mg), and ferric iron (Fe3+) in the second formula position. The formula amounts of silicon (Si) and oxygen (O) are the same for most monoclinic pyroxenes. For minerals such as jadeite and omphacite, where only a few elements substitute for each other, a continuum in chemical composition often exists that ranges from an end member with only one element in each formula position (e.g., jadeite, with only Na in the first position) to another end member with only the other element in that formula position (e.g., the Minerals 2021, 11, x FOR PEER REVIEW 24 of 34 pyroxene diopside has a formula of CaMgSi2O6, with only Ca in the first position). When related minerals can occur with any ratio of the end member elements in the first formula position (Na to Ca in this example), the chemical relationship between the minerals is mineralscalled a solidis called solution a solid series. solution If just series. two elementsIf just two are elements substituting, are substituting, the relationship the relation- between shipminerals between can minerals be defined can as be percentages defined as alongpercentages a mixing along line. a However,mixing line. in However, the case of in most the casepyroxenes, of most more pyroxenes, than two more elements than two are inelements play, requiring are in play, the use requiring of a triangular the use plot,of a trian- called gulara ternary plot, diagram, called a ternary to display diagram, multiple to element display substitutionsmultiple element simultaneously substitutions (Figure simultane- 25). In ouslythis type (Figure of diagram, 25). In this each type corner of diagram, represents each 100% corner of thatrepresents component. 100% of that component.

FigureFigure 25. 25. AA ternary ternary diagram diagram for for Na Na-bearing-bearing pyroxenes pyroxenes shows shows the the relationships relationships between between jadeite, jadeite, omphacite, and other related minerals. omphacite, and other related minerals. Chemically, omphacite is an intermediate composition in the solid solution series between the broad group of Ca,Mg,Fe-pyroxenes and end member jadeite (NaAlSi2O6). The omphacite compositional space, however, is very broad and the boundaries are not strictly defined. Most mineralogical references use 20% and 80% (Ca,Mg,Fe)Si2O6 component as the bounding horizontal lines for the omphacite region, as seen in Figure 25, but this leaves a very restricted space for jadeite that might not be representative of the material seen in the gem market.

10.2. Identification Using Raman Analysis Raman spectroscopy has proven to be an invaluable technique for the identification of minerals. When excited by a laser, most minerals emit light that is shifted in frequency relative to the wavelength of the laser. This shift is characteristic of the structure and chemistry of the mineral and can be used as an identification tool when compared to a database of known mineral shifts. In most cases, care has to be taken with the orientation of the crystals during Raman analysis because the technique is extremely sensitive to crystallographic directions. Fortunately, in the case of polycrystalline materials such as the jadeite and omphacite used in the jewelry industry, the tiny crystals are randomly oriented and do not significantly affect the Raman data. Although not commonly used in gemology for the separation of different pyroxenes, Raman spectroscopy shows distinct differences between jadeite and omphacite due to the elemental substitutions and corresponding structural changes that are very useful for identification (Figure 26). Minerals 2021, 11, 177 22 of 31

Chemically, omphacite is an intermediate composition in the solid solution series between the broad group of Ca,Mg,Fe-pyroxenes and end member jadeite (NaAlSi2O6). The omphacite compositional space, however, is very broad and the boundaries are not strictly deﬁned. Most mineralogical references use 20% and 80% (Ca,Mg,Fe)Si2O6 component as the bounding horizontal lines for the omphacite region, as seen in Figure 25, but this leaves a very restricted space for jadeite that might not be representative of the material seen in the gem market.

10.2. Identification Using Raman Analysis Raman spectroscopy has proven to be an invaluable technique for the identification of minerals. When excited by a laser, most minerals emit light that is shifted in frequency relative to the wavelength of the laser. This shift is characteristic of the structure and chemistry of the mineral and can be used as an identification tool when compared to a database of known mineral shifts. In most cases, care has to be taken with the orientation of the crystals during Raman analysis because the technique is extremely sensitive to crystallographic directions. Fortunately, in the case of polycrystalline materials such as the jadeite and omphacite used in the jewelry industry, the tiny crystals are randomly oriented and do not significantly affect the Raman data. Although not commonly used in gemology for the separation of different pyroxenes, Raman spectroscopy shows distinct differences Minerals 2021, 11, x FOR PEER REVIEWbetween jadeite and omphacite due to the elemental substitutions and corresponding25 of 34 structural changes that are very useful for identification (Figure 26).

FigureFigure 26. 26.Raman Raman spectra spectra collected collected with with a 514a 514 nm nm laser laser reveal reveal important important differences differences between between jadeite jade- andite omphacite.and omphacite. Data Data from from RRUFF RRUFF database database [58]. [58].

JadeiteJadeite shows shows prominent prominent Raman Raman peaks peaks at at ~204, ~204, 255, 255, 310, 310, 328, 328, 375, 375, 433, 433 524,, 524, 575, 575, 700, 700, 779,779, 987, 987, and and 10391039 cmcm−−11, ,whereas whereas omphacite omphacite has has distinct distinct features features at at335, 335, 374, 374, 409, 409, 554, 554, 678, 678,905, 905, and and 1015 1015 cm− cm1. While−1. While many many Raman Raman features features are slightly are slightly different different between between the miner- the minerals,als, the most the most prominent, prominent, and potentially and potentially useful useful (as shown (as shown in Figure in Figuress 26–28 26),– are28), the are peaks the peaksthat occur that occur at 678 at (omphacite) 678 (omphacite) and 700 and (jadeite) 700 (jadeite) cm−1.cm −1. As effective as Raman analysis is at separating jadeite and omphacite, it has tradi- tionally been used as a spot technique. High-pressure minerals such as jadeite and rocks that contain these minerals tend to be somewhat heterogeneous due to the nature of their geologic formation. As a result, a single Raman analysis in one spot of a gem may reveal a very different composition than another location. Raman mapping helps us to better understand the distribution of mineral phases, even those that are only subtly distinguishable by a shifted Raman peak, and evaluate the composition of a larger, more representative portion of a gem than a single spectrum might allow. For example, a Raman map of the peak position of the ~678–700 cm−1 feature in jadeite/omphacite shows a very heterogeneous growth pattern of interlocking grains of nearly pure jadeite and mixed jadeite/omphacite (Figure 27).

Figure 27. Raman map of a “jadeite” ring (left) shows a mixture of mineral compositions. The false color map (right) ranges in color from jadeite (pink) to omphacite (blue) based on the position of the 678–700 cm−1 primary Raman peak. Minerals 2021, 11, x FOR PEER REVIEW 27 of 36

able by a shifted Raman peak, and evaluate the composition of a larger, more representative portion of a gem than a single spectrum might allow. For example, a Raman map of Minerals 2021, 11, x FOR PEER REVIEW the peak position of the ~678–700 cm−1 feature in jadeite/omphacite shows a very hetero-27 of 36 geneous growth pattern of interlocking grains of nearly pure jadeite and mixed jadeite/omphacite (Figure 27).

able by a shifted Raman peak, and evaluate the composition of a larger, more representative portion of a gem than a single spectrum might allow. For example, a Raman map of the peak position of the ~678–700 cm−1 feature in jadeite/omphacite shows a very hetero- Minerals 2021, 11, 177 geneous growth pattern of interlocking grains of nearly pure jadeite and mixed jade-23 of 31 ite/omphacite (Figure 27).

Under higher magnification with the Raman mapping microscope, the different mineral grains in the same sample can be seen with reflected light. A high-resolution Raman map of individual and adjacent grains shows that the nearly pure jadeite composition is concentrated at the edges of the grains, with the matrix between grains being largely com- Figure 27. Raman map of a “jadeite”posed ringof omphacite (left) shows (Figure a mixture 28). This of mineral level of compositions. detail helps us The to better false color understand map (right the) com- ranges Figure 27. Raman map of a “jadeite” ring (left) shows a mixture of mineral compositions. The false color map (right) ranges plexity of the geological history for this apparently uniform−1 green material. inin color from from jadeite jadeite (pink) (pink) to to omphacite omphacite (blue) (blue) based based on on the the position position of the of the678–700 678–700 cm−1 cm primaryprimary Raman Raman peak. peak.

-1 FigureFigure 28. High-resolution 28. High-resolution Raman Raman mapping mapping of of the the Raman Raman shift shiftposition position (cm−) 1within) within a small a small area area of the of thering ring from from Figure Figure 27 27 shows jadeite concentrated at grain edges and a matrix of omphacite in between grains. The false color map ranges in shows jadeite concentrated at grain edges and a matrix of omphacite in between grains. The false color map ranges in color color from jadeite (pink) to omphacite (blue) based on the position of the 678–700 cm−1 primary Raman peak. from jadeite (pink) to omphacite (blue) based on the position of the 678–700 cm−1 primary Raman peak.

As effective as Raman analysis is at separating jadeite and omphacite, it has tradition- ally been used as a spot technique. High-pressure minerals such as jadeite and rocks that contain these minerals tend to be somewhat heterogeneous due to the nature of their geologic formation. As a result, a single Raman analysis in one spot of a gem may reveal a very Figure 28. High-resolution Ramandifferent mapping composition of the Raman than shift another position location. (cm-1) within Raman a small mapping area of helps the ring us from to better Figure understand 27 shows jadeite concentratedthe at grain distribution edges and of a matrix mineral of omphacite phases, even in between those grains. that are The only false subtlycolor map distinguishable ranges in by a color from jadeite (pink) to omphaciteshifted Raman (blue) based peak, on and the position evaluate of the the composition678–700 cm−1 primary of a larger, Raman more peak. representative portion of a gem than a single spectrum might allow. For example, a Raman map of the peak position of the ~678–700 cm−1 feature in jadeite/omphacite shows a very heterogeneous growth pattern of interlocking grains of nearly pure jadeite and mixed jadeite/omphacite (Figure 27). Under higher magniﬁcation with the Raman mapping microscope, the different mineral grains in the same sample can be seen with reﬂected light. A high-resolution Raman map of individual and adjacent grains shows that the nearly pure jadeite composition is concentrated at the edges of the grains, with the matrix between grains being largely composed of omphacite (Figure 28). This level of detail helps us to better understand the complexity of the geological history for this apparently uniform green material. Minerals 2021, 11, x FOR PEER REVIEW 28 of 36

Minerals 2021, 11, 177 24 of 31

While many “jadeite” jewelry pieces in the trade consist of a mixed composition, some have been found to be nearly completely composed of omphacite, as shown in Fig- ure 29. WhileTraditional many Raman “jadeite” spectroscopy jewelry pieces can in identify the trade the consist differences of a mixed between composition, jadeite and some relatedhave pyroxenes been found such to be as nearly omphacite; completely howeve composedr, Raman of mapping omphacite, allows as shown for a more in Figure thor- 29. oughTraditional understanding Raman of spectroscopy the true composition can identify of gems the differencesas a whole and between thus provides jadeite and a better related representationpyroxenes such of what as omphacite; a consumer however, is purchasing. Raman mapping allows for a more thorough understanding of the true composition of gems as a whole and thus provides a better representation of what a consumer is purchasing.

FigureFigure 29. 29. RamanRaman mapping mapping of ofjadeite/omphacite jadeite/omphacite composition composition from from one one stone stone in the in thering ring seen seen on onthe the left left shows shows a mostly a mostly uniformuniform composition composition of ofomphacite. omphacite. This This ring ring would would most most often often be besold sold as jadeite as jadeite in the in the trade trade due due to toa relatively a relatively poor poor understanding of the compositional differences and the gemological difficulty in separating them. The false color map understanding of the compositional differences and the gemological difficulty in separating them. The false color map (right) (right) ranges in color from jadeite (pink) to omphacite (blue) based on the position of the 678–700 cm−1 primary Raman ranges in color from jadeite (pink) to omphacite (blue) based on the position of the 678–700 cm−1 primary Raman peak. peak. 11. Raman Mapping of Pearls 11. Raman Mapping of Pearls Pearls are mainly composed of calcium carbonate (CaCO3) biominerals together with organicPearls matrix are mainly and water. composed In pearl of calcium analysis, carbonate Raman spectroscopy (CaCO3) biominerals is a rapid, together non-destructive with organictechnique matrix that and mainly water. is usedIn pearl to identifyanalysis, different Raman CaCOspectroscopy3 polymorphs, is a rapid, including non-destruc- aragonite, tivecalcite, technique and vaterite,that mainly and is determine used to identify color origin different of the CaCO pearl:3 polymorphs, whether it is including caused by arag- natural onite,pigments calcite, (in and particular, vaterite, and polyenes determine and uroporphyrin)color origin of the or has pearl: been whether modified it is bycaused treatment by naturalprocesses pigments [5,59– 61(in]. particular, polyenes and uroporphyrin) or has been modified by treatmentAragonite processes is the[5,59–61]. most common CaCO3 polymorph found in nacreous pearls of both saltwaterAragonite (SW) is the and most freshwater common (FW) CaCO environments,3 polymorph asfound well in as nacreous in various pearls non-nacreous of both saltwaterpearls that (SW) exhibit and freshwater surface flame (FW) structure environments, and porcelain-like as well as luster in various (known non-nacreous as porcelaneous pearlspearl), that such exhibit as conchsurface and flame melo structure pearls [an62d]. porcelain-like Calcite is known luster to (known be a main as porcelane- composition ousof pearl), some non-nacreoussuch as conch and pearls melo produced pearls [62]. in some Calcite mollusk is known species to be of a Pinnidae,main composition Pectinidae, of andsome Ostreidae non-nacreous families pearls [63 –produced65]. Moreover, in some some mollusk marine species nacreous of Pinnidae, pearls can Pectinidae, develop in andthe Ostreidae pearl sac families that originated [63–65] from. Moreover, young epitheliumsome marine cells, nacreous which ispearls composed can develop of prismatic in thelayers pearl ofsac calcite that originated in the center from and young layers epithelium of aragonite cells, deposition which is at composed the outside of [ 66prismatic]. Vaterite layersis an of unstable calcite in CaCOthe center3 polymorph and layers that of ar usuallyagonite isdeposition not found at inthe the outside pearl, [66]. especially Vaterite for is anmarine unstable pearls. CaCO Nevertheless,3 polymorph the that occurrence usually is ofnot vaterite found hasin the been pearl, reported especially on the for surface ma- rineof pearls. freshwater Nevertheless, cultured nacreousthe occurrence pearls inof thevaterite white has frosty been region, reported which on the is lusterless surface of and freshwaterinfluences cultured the quality nacreous of pearls pearls [67 –in70 ].the Vaterite white hasfrosty also region, been discoveredwhich is lusterless in the center and of influencesfreshwater the non-bead quality of cultured pearls [67–70]. pearls, whichVateri mayte has relate also tobeen implanted discovered tissues in the that center were usedof freshwaterto culture non-bead the pearls cultured [68,71]. pearls, Therefore, which characterizing may relate to CaCO implanted3 polymorphs tissues isthat a valuable were usedtechnique to culture which the pearls can help [68,71]. in determining Therefore, the characterizing mollusk species, CaCO environment,3 polymorphs and is a origin valu- of ablecertain technique types which of pearls can in help gemological in determini laboratories.ng the mollusk species, environment, and origin ofRaman certain mapping types of pearls is not routinelyin gemological used laboratories. for pearl testing, yet the technique is being investigatedRaman mapping for its abilityis not routinely to assist inused verifying for pearl the testing, environmental yet the technique growth condition is being of investigateda questionable for its pearl ability that to exhibitsassist inunusual verifying chemical the environmental characteristics growth and condition internal growth of a questionablestructures. pearl This pearlthat exhibits has been unusual reported chemical previously, characteristics with its growth and internal environment growth and origin inconclusive (pearl A in [72]). Subsequently, to verify the pearl’s identity, it was cut in half to analyze in more detail. A diamond-plated saw was used to cut the pearl in a direction relatively parallel to the flat base, and the surfaces of both cross-sections Minerals 2021, 11, x FOR PEER REVIEW 27 of 34

11. Raman Mapping of Pearls

Pearls are mainly composed of calcium carbonate (CaCO3) biominerals together with organic matrix and water. In pearl analysis, Raman spectroscopy is a rapid, non-destructive technique that mainly is used to identify different CaCO3 polymorphs, including aragonite, calcite, and vaterite, and determine color origin of the pearl: whether it is caused by natural pigments (in particular, polyenes and uroporphyrin) or has been modified by treatment processes [5,59–61]. Aragonite is the most common CaCO3 polymorph found in nacreous pearls of both saltwater (SW) and freshwater (FW) environments, as well as in various non-nacreous pearls that exhibit surface flame structure and porcelain-like luster (known as porcelaneous pearl), such as conch and melo pearls [62]. Calcite is known to be a main composition of some non-nacreous pearls produced in some mollusk species of Pinnidae, Pectinidae, and Ostreidae families [63–65]. Moreover, some marine nacreous pearls can develop in the pearl sac that originated from young epithelium cells, which is composed of prismatic layers of calcite in the center and layers of aragonite deposition at the outside [66]. Vaterite is an unstable CaCO3 polymorph that usually is not found in the pearl, especially for marine pearls. Nevertheless, the occurrence of vaterite has been reported on the surface of freshwater cultured nacreous pearls in the white frosty region, which is lusterless and influences the quality of pearls [67–70]. Vaterite has also been discovered in the center of freshwater non-bead cultured pearls, which may relate to implanted tissues that were used to culture the pearls [68,71]. Therefore, characterizing CaCO3 polymorphs is a valuable technique which can help in determining the mollusk species, environment, and origin of certain types of pearls in gemological laboratories. Raman mapping is not routinely used for pearl testing, yet the technique is being investigated for its ability to assist in verifying the environmental growth condition of a questionable pearl that exhibits unusual chemical characteristics and internal growth structures. This pearl has been reported previously, with its growth environment and

Minerals 2021, 11, 177 origin inconclusive (pearl A in [72]). Subsequently, to verify the pearl’s identity, it was25 of cut 31 in half to analyze in more detail. A diamond-plated saw was used to cut the pearl in a direction relatively parallel to the flat base, and the surfaces of both cross-sections were left unpolished for the study. The study presented here only focuses on the top cross- weresection left sample, unpolished which for weighed the study. 4.78 Thecarats study and measured presented 11.81 here × only 10.95 focuses × 5.06 mm on the (Figure top cross-section30). The sample’s sample, chemical which weighedcomposition 4.78 was carats examined and measured using LA 11.81-ICP×-MS10.95 analysis.× 5.06 mmTwo (Figurelines of 30 54). Thespots sample’s were analyzed chemical across composition the surface was examined at perpendicular using LA-ICP-MS directions analysis. to each Twoother. lines The of trace 54 spots element were data analyzed revealed across that the some surface spots at in perpendicular the central area directions contained to each very other.high magnesium The trace element (Mg) concentrations data revealed (761 that– some3360 ppm) spots compared in the central with area regular contained SW (around very high100– magnesium300 ppm) and (Mg) FW concentrations (below 100 ppm) (761–3360 values, ppm) and comparedtheir manganese with regular (Mn), SWbarium (around (Ba), 100–300strontium ppm) (Sr), and and FW sodium (below (Na) 100 values ppm) values,did not andcorrelate their manganeseto either SW (Mn), or FW barium characteris- (Ba), strontiumtics. (Sr), and sodium (Na) values did not correlate to either SW or FW characteristics.

Figure 30. Unpolished surface of the top cross-section sample with two perpendicular lines of spot Figure 30. Unpolished surface of the top cross-section sample with two perpendicular lines of spot analyzed by LA-ICP-MS analysis. analyzed by LA-ICP-MS analysis.

Raman spectroscopy analyses on the cross-section of this specimen were conducted using a Renishaw inVia Raman microscope with an 830 nm diode laser excitation wavelength at room temperature. The 830 nm laser provided better peak resolution compare to the 514 nm laser that is normally used in pearl testing owing to higher background ﬂuorescence in this sample. The results indicated that the high Mg value spots are composed of vaterite, which could be an indication of growth under FW environment. In addition, aragonite and calcite spectra were also found in nearby areas together with vaterite. The three CaCO3 2− polymorphs can be distinguished by different band positions of carbonate ion (CO3 ) modes. Raman spectra of vaterite showed major bands at 1075, 1080, and 1091 cm−1 which 2− correspond to symmetric stretching (V1) mode of CO3 , and minor bands at 667, 685, −1 2− 740, 743, and 751 cm which correspond to in-plane bending (V4) modes of CO3 . The results conformed with previous ﬁndings [68]. Raman bands of aragonite were observed −1 −1 at 1085 cm for the V1, and doublet bands at 701 and 705 cm for the V4. The presence of −1 calcite was indicated by the bands at 1086 (V1), and 712 cm (V4). Though some dominant vaterite spectra contain a tiny band of aragonite at 702 cm−1, the vaterite bands at 685, 740, 743, 751, and 1075 cm−1 (shoulder) also are present in the dominant aragonite calcite spectra (Figure 31). Raman mapping was further used to determine the distribution of the different polymorphs with 785 nm laser excitation wavelength at room temperature, 0.04 s exposure time per pixel, 20 µm pixel size, and a total of 10 scans. The Raman map in Figure 32A showed intensity variation at 1091 cm−1, the dominant peak for vaterite. Raman mapping of the 1091 cm−1 peak indicated a vaterite zone in the center (yellow to red in the false color map of Figure 32A) and the surrounding low intensities (dark blue) consisted of aragonite. However, the region between aragonite and calcite in the central area close to vaterite cannot be separated using this peak position. The Raman map of the calcite −1 band at 712 cm in Figure 32B displayed the distribution areas of the CaCO3 polymorphs better: calcite was only detected in an irregular, small area (green in false color map of Figure 32B) associated with vaterite. The vaterite growth sector in the maps corresponded to the pearl’s internal structure, obtained from X-ray computed microtomography analysis (µ-CT) and observed with photomicrography in transmitted light (Figure 33). The vaterite Minerals 2021, 11, x FOR PEER REVIEW 28 of 34

Minerals 2021, 11, 177 26 of 31 Raman spectroscopy analyses on the cross-section of this specimen were conducted using a Renishaw inVia Raman microscope with an 830 nm diode laser excitation wavelength at room temperature. The 830 nm laser provided better peak resolution compare showed lower radiodensity (i.e., more transparent) to X-rays than the aragonite area, and it appearedto the 514 as nm darker laser gray that inis thenormallyµ-CT image. used in pearl testing owing to higher background fluorescenceThe mixture in this of aragonite–vateritesample. The results formation indicated inthat the the central high areaMg value of freshwater spots are pearls com- wasposed previously of vaterite studied, which and could reported be an [i68ndication,71,73]; however,of growth the under authors FW didenvironment. not detect calciteIn ad- indition, their aragonite studies. Previousand calcite researchers spectra were reported also found large in domains nearby ofareas calcite together together with with va- aragoniteterite. The in three the centralCaCO3 areapolymorphs of non-bead can be cultured distinguished freshwater by pearlsdifferent [74 band]. However, positions they of 2− believedcarbonate that ion the(CO combination3 ) modes. Raman of aragonite, spectra of vaterite, vaterite and showed calcite major formation bands at in 1075, thesame 1080, −1 2− freshwaterand 1091 cm pearl which had nevercorrespond been reportedto symmetric in the stretching literature (ʋ to1) date.mode The of CO detection3 , and ofminor the vateritebands at area 667, in 685, the 740, center 743, of and the 751 pearl cm by−1 which Raman correspond mapping technique to in-plane helped bending to verify(ʋ4) modes that theof CO pearl32−. wasThe likelyresults from confo freshwaterrmed with environment previous findings since vaterite [68]. wasRaman not reportedbands of toaragonite present inwere saltwater observed pearls at 1085 [71 ,cm75,−761 for]. the In accordance ʋ1, and doublet with bands the previous at 701 and studies, 705 cm it−1 isfor potentially the ʋ4. The apresence non-bead of calcite cultured was pearl. indicated Nevertheless, by the bands a recent at 1086 study (ʋ1), reportedand 712 cm a vaterite−1 (ʋ4). Though area on some the surfacedominant of avaterite natural spectra freshwater contain pearl a [tiny77]. Furtherband of studyaragonite on the at bottom702 cm− cross-section1, the vaterite sample bands andat 685, another 740, 743, pearl 751, that and displayed 1075 cm similar−1 (shoulder) unusual also chemical are present characteristics in the dominant (pearl aragonite B in [72]) hascalcite to bespectra performed (Figure to 31). ensure the identiﬁcation results.

Raman Spectra Raman Spectra Vaterite A Vaterite 230,000 1,200,000 712 Aragonite + vaterite Aragonite + vaterite B Calcite + vaterite Calcite + vaterite 1,000,000 190,000

800,000 701 705 150,000 743 740 751 600,000 110,000 668 685 400,000 INTENSITY INTENSITY (COUNTS) 70,000

200,000 (COUNTS) INTENSITY

0 30,000 100 400 700 1000 1300 1600 660 680 700 720 740 760 780 -1 RamanRAMAN SHIFT(cmSpectra ) RAMAN SHIFT(cm-1) Vaterite 1085 C Aragonite + vaterite 1,200,000 Calcite + vaterite 1,000,000

800,000 1086 1091

600,000 1080 400,000 1075 INTENSITY (COUNTS) INTENSITY 200,000

0 1030 1050 1070 1090 1110 1130 RAMAN SHIFT(cm-1)

FigureFigure 31.31. ((AA)) RamanRaman spectraspectra ofof vateritevaterite (orange),(orange), dominantdominant aragonitearagonite withwith minorminor vateritevaterite bandsbands (blue),(blue), dominantdominant calcitecalcite withwith minorminor vaterite bands (green). (green). (B (B) )The The most most intense intense bands bands correspond correspond to toʋ1 ofV1 vateriteof vaterite showed showed at 1075, at 1075, 1080, 1080, and 1091 and −1 −1 −1 −1 −1 −1 1091cm cmwhereaswhereas in aragonite in aragonite and calcite and calcite spectra spectra displayed displayed at 1085 at cm 1085 and cm 1086and cm 1086, respectively. cm , respectively. (C) The (ʋC4) o Thef vateriteV4 of vateriteshowed showed as weak as bands weak at bands 667, at685, 667, 740, 685, 743, 740, and 743, 75 and1 cm 751−1. cmDoublet−1. Doublet bands bandsat 701 atand 701 705 and cm 705−1 belong cm−1 belong to aragonite to aragonite and a −1 andsingle a single band bandat 712 at cm 712 cmindicates−1 indicates calcite. calcite. The mixture The mixture of different of different polymorphs polymorphs in the in same the same spectra spectra was waspresented. presented. The dominant vaterite spectrum contain a tiny band of aragonite at 702 cm−1, and the vaterite bands at 685, 740, 743, 751, and The dominant vaterite spectrum contain a tiny band of aragonite at 702 cm−1, and the vaterite bands at 685, 740, 743, 751, 1075 cm−1 (shoulder) are presented in the dominant aragonite calcite spectra. and 1075 cm−1 (shoulder) are presented in the dominant aragonite calcite spectra. Minerals 2021, 11, x FOR PEER REVIEW 31 of 36 Minerals 2021, 11, 177 27 of 31

Figure 32. Raman maps collected using the 785 nm laser at room temperature showed intensity variation of the vaterite − − − Figureband at 32. 1091 Raman cm 1 maps(A) and collected the calcite using band the at 785 712 nm cm laser1 (B ).at Raman room te mapmperature at 1091 cmshowed1 peak intensity position variation showed highof the intensities vaterite Minerals 2021, 11, x FOR− 1PEER REVIEW −1 −1 30 of 34 bandof vaterite at 1091 zone cm in (A the) and center theand calcite enclosing band at with 712 lowcm intensities (B). Raman (dark map blue) at 1091 of aragonitecm peak area. position Calcite showed wasclearly high intensities indicated ofwith vaterite 712 cm zone−1 peakin the position center and as anenclosing irregular with small low area intensit (greenies intensity) (dark blue) associated of aragonite with area. vaterite Calcite in the was center. clearly indicated with 712 cm−1 peak position as an irregular small area (green intensity) associated with vaterite in the center.

The mixture of aragonite–vaterite formation in the central area of freshwater pearls was previously studied and reported [68,71,73]; however, the authors did not detect calcite in their studies. Previous researchers reported large domains of calcite together with aragonite in the central area of non-bead cultured freshwater pearls [74]. However, they believed that the combination of aragonite, vaterite, and calcite formation in the same freshwater pearl had never been reported in the literature to date. The detection of the vaterite area in the center of the pearl by Raman mapping technique helped to verify that the pearl was likely from freshwater environment since vaterite was not reported to present in saltwater pearls [71,75,76]. In accordance with the previous studies, it is potentially a non-bead cultured pearl. Nevertheless, a recent study reported a vaterite area on the surface of a natural freshwater pearl [77]. Further study on the bottom cross-section sample and another pearl that displayed similar unusual chemical characteristics (pearl B in [72]) has to be performed to ensure the identification results.

Figure 33. The vaterite growth sector presented in the maps corresponds to the pearl’s internal structure, obtained from Figure 33. The vaterite growth sector presented in the maps corresponds to the pearl’s internal structure, obtained from X-ray computed microtomography analysis (µ-CT, left) and observed in photomicrograph with transmitted light (right). µ left right TheX-ray vaterite computed showed microtomography lower radiodensity analysis (more ( -CT,transparent)) and to observed X-ray than in photomicrograph the aragonite area, with and transmitted it appeared lightas a (darker). grayThe vateriteregion in showed µ-CT image. lower radiodensity (more transparent) to X-ray than the aragonite area, and it appeared as a darker gray region in µ-CT image. 12. Future Possibilities and Conclusions 12. Future Possibilities and Conclusions The advancements of various treatments and laboratory-growth methods have re- The advancements of various treatments and laboratory-growth methods have re- quiredquired the useuse ofof complex complex analytical analytical identiﬁcation identification instruments, instruments, such such as mappingas mapping spectrom- spec- trometerseters and automatedand automated gem gem testing. testing. The The ability ability to quickly to quickly and and easily easily collect collect Raman Raman and and PL PLdata data has has allowed allowed gemologists gemologists to perform to perform fundamental fundamental research research on spatial on spat inhomogeneitiesial inhomoge- neitiesin ways in thatways were that notwere possible not possible just a just few a shortfew short years years ago. ago. This This has has created created numerous numer- ousavenues avenues across across a wide a wide variety variety of of gem gem materials materials to to study study growth growth mechanisms,mechanisms, aa deeper exploration of various treatments, and look into wider applications such as geographic origin determination. In the future, we expect to see improvements in data resolution and sensitivity and likely the integration of various mapping technologies. While this article has focused on several gemological applications and scientific research using Raman and PL mapping, other applications on gem materials not recounted here have also been studied [78–84]. Additionally, similar research aims have involved other mapping technologies such as IR absorption mapping [85–89], cathodoluminescence mapping [90–95], chemical analysis using LIBS [96] or X-ray fluorescence [97], electron paramagnetic resonance (EPR) imaging [98], and fluorescence decay (luminescence lifetime) mapping [99,100]. In the future, we expect that instrumentation will be able to integrate these various technologies together. The Raman and PL mapping technique allows scientists and researchers to quickly collect and analyze Raman and PL spectra so they can more easily identify the nature of defects, species identification, or structure in diamonds, pearls, and other gemstones. Ad- ditionally, it permits gemologists to evaluate new criteria for distinguishing treated and lab-grown gemstones and, for some, to better discern the country of origin.

Author Contributions: Conceptualization, S.E.-M., C.M.B., A.C.P., A.H., Z.S. and G.M.; investigation, S.E.-M., C.M.B., A.C.P., A.H., Z.S. and G.M.; data curation, S.E.-M., C.M.B., A.C.P., A.H., Z.S. and G.M.; writing—original draft preparation, S.E.-M., C.M.B., A.C.P., A.H., Z.S. and G.M.; writing—review and editing, S.E.-M.; project administration, S.E.-M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Minerals 2021, 11, 177 28 of 31

exploration of various treatments, and look into wider applications such as geographic origin determination. In the future, we expect to see improvements in data resolution and sensitivity and likely the integration of various mapping technologies. While this article has focused on several gemological applications and scientific research using Raman and PL mapping, other applications on gem materials not recounted here have also been studied [78–84]. Additionally, similar research aims have involved other mapping technologies such as IR absorption mapping [85–89], cathodoluminescence mapping [90–95], chemical analysis using LIBS [96] or X-ray fluorescence [97], electron paramagnetic resonance (EPR) imaging [98], and fluorescence decay (luminescence lifetime) mapping [99,100]. In the future, we expect that instrumentation will be able to integrate these various technologies together. The Raman and PL mapping technique allows scientists and researchers to quickly collect and analyze Raman and PL spectra so they can more easily identify the nature of defects, species identification, or structure in diamonds, pearls, and other gemstones. Additionally, it permits gemologists to evaluate new criteria for distinguishing treated and lab-grown gemstones and, for some, to better discern the country of origin.

Author Contributions: Conceptualization, S.E.-M., C.M.B., A.C.P., A.H., Z.S. and G.M.; investigation, S.E.-M., C.M.B., A.C.P., A.H., Z.S. and G.M.; data curation, S.E.-M., C.M.B., A.C.P., A.H., Z.S. and G.M.; writing—original draft preparation, S.E.-M., C.M.B., A.C.P., A.H., Z.S. and G.M.; writing— review and editing, S.E.-M.; project administration, S.E.-M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors thank Lorne Loudin, Troy Ardon, Evan Smith, Barbara Dutrow, Wuyi Wang, Chunhui Zhou, and Ulrika D’Haenens-Johansson for their advice and assistance with the experimental collection and/or data analysis. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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