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CVD Synthetic Diamond Identifed in the GIT-GTL

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CVD Synthetic Diamond Identifed in the GIT-GTL Lab Update CVD Synthetic Diamond Identified in the GIT-GTL Supparat Promwongnan, Thanong Leelawatanasuk, Thanapong Lhuaamporn of the GIT-Gem Testing Laboratory Keywords: CVD, HPHT, Synthetic, Diamond, Color enhancement, spectroscopy, treatment Figure 1. A total of 27 faceted slightly tinted yellow CVD-grown synthetic diamonds were sent to the GIT-GTL for diamond grading reports, as of natural origin. The samples selected have an average weight of 0.40 ct. Photo by S. Promwongnan. Introduction – infrared absorption spectra showing the of these samples ranged from VVS2 to SI1. During the past decade, a large number of presence of trace amounts of nitrogen The colour grade of the diamonds is slightly gem-quality synthetic diamonds have entered defect at 1344 cm−1; tinted yellow. All samples were further the diamond industry with a wide range of – photoluminescence spectra showing strong tested with the D-Screen which is a portable quality in terms of size, colour and clarity. emissions from N-V centres and distinct diamond screening device. The internal Consequently, there have been reports of cases doublet peaks at 736.6 nm and 736.9 nm, features were observed under a gemmologi- of fraud in the diamond business, for example: related to the Si-related defect; cal microscope. Furthermore between crossed intentionally selling synthetic diamond as – the lamellar growth patterns seen with the polarizers, strain patterns and interference natural diamond or intentionally mixing a DiamondView imaging system. colours could be seen. All photomicrographs were collected by a Canon EOS 7D Photo- natural diamond parcel with a synthetic one. All these characteristics can be used to sepa- microscope. Fluorescence reactions were Further, a significant number of melee sized rate the CVD-grown synthetic diamonds from observed using a gemmological UV lamp in synthetic diamonds have flooded the market- natural diamonds. Moreover, we also observed both long-wave (365 nm) and short-wave place raising concern in the diamond sector features indicative of high temperature-high (254 nm) ultraviolet light. More observation of at both domestic and international levels. pressure treatment (HPHT) for CVD-grown growth patterns – photoluminescence images These unethical trade practices have brought a synthetic diamonds such as the lack of were investigated using the DTC Diamond. negative impact to consumer confidence in the H-related peaks at 3123 cm-1 and 3323 cm-1 in View instrument (wavelength <230 nm). diamond industry as a whole. the mid-IR range and the photoluminescence A Thermo Nicolet 6700 Fourier-transform in- In November 2015, a parcel of slightly emission peak at 596.5/597.0 nm doublet, frared (FTIR) spectrometer was used to record tinted yellow diamonds was submitted to the as commonly observed in post-grown CVD IR transmittance spectra in the mid-IR range Gem and Jewelry Institute of Thailand’s Gem synthetic diamonds. Testing Laboratory (GIT-GTL) for a diamond (6000–400 cm−1) with a resolution of 2.0 cm–1 grading report. What was somewhat surprising Samples and Procedures and 250 scans to determine the diamond was that twenty-seven samples in this suite The 27 round brilliant cut CVD-grown type. Photoluminescence (PL) spectroscopy proved to be Chemical Vapour Deposition synthetic diamonds shown in Figure 1, were was performed on all samples while they (CVD-grown) synthetic diamonds (Figure 1). sent to the GIT-GTL for diamond grading were immersed in liquid nitrogen using a These samples exhibit the typical reports as being of natural origin. They ranged Renishaw inVia Raman micro-spectrometer characteristics of the CVD-grown synthetic in size from 0.39 to 0.44 ct with approximately with the He-Cd laser (325 nm) and Nd:YAG diamond exhibiting: 4.6–4.9 mm in diameter. The clarity grade laser (532 nm). – small concentrations of isolated nitrogen (N) impurities; Lab Update: CVD Synthetic Diamond Identified in the GIT-GTL | 27 – 32 28 Results Microscopic Features The clarity grade of this suite ranged from VVS2 to SI1. Observation with Gemmological Observation a gemmological microscope showed several samples contained very Based on the observed gemmological properties, these samples were small, opaque, dark crystals (Figure 2a) and some samples had a number consistent with being diamond with confirmation provided by the of tiny pinpoint-like inclusions (Figure 2b). Many samples had a small, diamond tester. And to make sure of this outcome a D-screen device frosted cavity reaching to the surface on the pavilion (Figure 2c-d) with was used to verify the result. However, surprisingly all the samples a few diamonds containing parallel internal growth lines (Figure 2e-f). examined in the study received a “Passed” (green indicators) tag which Observation of such internal graining could be considered an indication suggested that they were all natural diamond. of the CVD-grown synthetic diamond. Moreover nicks, cavities, extra facets and scratches were observed as the external inclusions. a b c d e f Figure 2. Examined under the microscope: a (No. 152210), the internal inclusion features consist of very small dark crystals. b (No. 152234), Fibre optic light illumination revealed a number of very fine pinpoint-like inclusions. c (No. 152223), Note, frosted cavity reaching to the surface on the pavilion side and d (No. 152234) the girdle of the diamond visible through the crown side. In e & f (No. 152223 internal graining is exhibited. Photomicrographs by S. Promwongnan; field of view 1.4, 2.5, 1.2, 1.5, 2.3 and 1.8 mm for photos a–f, respectively. The Australian Gemmologist | 2016 | Volume 26, Number 1 – 4 29 Further examination with a microscope with cross-polarized light revealed – three samples fluoresced very weak red; and, internal features like the cross-hatched bands (tatami) with irregular – five samples fluoresced very weak chalky yellowish green. bands of interference colours in some areas of all samples (Figure 3a-c). None of the samples exhibited any phosphorescence to standard These results are consistent with the presence of very low nitrogen UV lamps. concentration in type Ib diamonds. After exposure to the deep ultraviolet wavelengths under the Dia- Fluorescence & DiamondView mondView, fluorescence imaging of all the samples revealed uneven blue fluorescence patterns on a background of yellowish green. Twenty-three The reactions to ultraviolet radiation were checked in both long-wave of the samples tested show distinctive lamellar growth patterns that and short-wave ultraviolet light. All samples were inert to long-wave UV are features typical for CVD synthetic diamond (Figure 4a-c). Phospho- radiation but fluoresced very weak colour to short-wave UV: rescence was also seen under the DiamondView instrument; weak to – thirteen samples fluoresced very weak orange; strong greenish blue coloured phosphorescence was also observed in – six samples fluoresced very weak reddish orange; twenty-four samples, weak yellowish green colored phosphorescence in one sample and no phosphorescence in two samples. a a b b c c Figure 3. The cross-hatched bands (tatami) with irregular bands of Figure 4. With a DiamondView, distinctive straight-to wavy lamellar growth interference colours seen with crossed polarizers on all samples investigated. striations were seen in most samples. Photomicrographs by S. Promwongnan; field of view 3.6 mm for photos a–c Photos by S. Promwongnan; Image width 4.0, 3.3, and 4.5 mm for photos a–c (No. 152201, 152205, 152224), respectively. (No. 152204, 152223, 152220), respectively. Lab Update: CVD Synthetic Diamond Identified in the GIT-GTL | 27 – 32 30 Advanced Instrument Analysis Spectra FTIR analysis identified absorption in the mid-IR region of all samples as type Ib diamond exhibiting absorption due to isolated nitrogen impurities, and characterized by a feature at 1344 cm−1 (Figure 5). The Hydrogen-related absorption features at 3107 cm−1 were also detected in fifteen samples. There was no H-related evidence of 3123 cm-1 and 3323 cm-1 in all samples examined. Figure 5. FTIR transmittance spectra from most samples showing an absorption of isolated N defects at 1344 cm−1 which were classified as type Ib diamonds. On the top-right, absorption transmittance spectrum of sample No. 152221 represented clearly a N-related defect. 30000 3000 575 PhotoluminescenceA spectra B 503 2500 25000 30000 3000 B 575 y y 2000 A 20000 t t 1: 152109 i i 503 s s 2500 25000 2: 152119 n n 3: 152120 e e t t 1500 15000 4: 152121 n n y y 2000 20000 I I t t 1: 152109 5: 152122 i i s s 2: 152119 6 152126 n 1000n 10000 3: 152120 7: 152201 e e t t 1500 15000 4: 152121 8: 152202 n n I I 59:: 115522122024 500 5000 6 152126 1000 10000 10: 152205 7: 152201 11: 152209 8: 152202 12: 152210 0 0 9: 152204 500 5000 13: 152211 412 413 414 415 416 500 510 520 530 540 550 560 570 580 10: 152205 14: 152212 11: 152209 15: 152213 Wavelength [nm] Wavelength [nm] 12: 152210 0 0 16: 152214 412 413 414 415 416 500 510 520 530 540 550 560 570 580 13: 152211 18000 30000 1147: :1 1552222120 C Wavelength [nm] D Wavelength [nm] 1158: :1 15522221231 1169: :1 15522221242 20: 152223 15000 18000 2500030000 17: 152220 C D 1281: :1 15522222214 19: 152222 22: 152234 y y 25000 2203: :1 1552222235 t 20000 t 15000 12000 i i 21: 152224 s 24: 152236 s n n 22: 152234 25: 152237 y e y e 23: 152235 t t 20000 t t 15000 26: 152238 12000 i 9000i n n s 24: 152236 s I 27: 152239 I n n 25: 152237 e e t t 15000 26: 152238 9000 10000 n n 6000 I 27: 152239 I 10000 6000 5000 3000 5000 3000 590 591 592 593 594 595 596 597 598 599 600 734 736 738 590 591 5W92 av59e3 len594gth5 9[5nm59]6 597 598 599 600 734 Wavele7n36gth [nm] 738 Wavelength [nm] Wavelength [nm] Figure 6.
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