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DIAMOND Natural Colorless Type Iab Diamond with Silicon-Vacancy

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DIAMOND Natural Colorless Type Iab Diamond with Silicon-Vacancy Editors Thomas M. Moses | Shane F. McClure DIAMOND logical and spectroscopic features con- Natural Colorless Type IaB firmed the diamond’s natural origin, – Diamond with Silicon-Vacancy despite the occurrence of [Si-V] emis- Defect Center sions. No treatment was detected. Examination of this stone indicated The silicon-vacancy defect, or [Si-V]–, that the [Si-V]– defect can occur, albeit is one of the most important features rarely, in multiple types of natural dia- in identifying CVD synthetic dia- monds. Therefore, all properties should monds. It can be effectively detected be carefully examined in reaching a using laser photoluminescence tech- conclusion when [Si-V]– is present. nology to reveal sharp doublet emis- sions at 736.6 and 736.9 nm. This Carmen “Wai Kar” Lo defect is extremely rare in natural dia- monds (C.M. Breeding and W. Wang, “Occurrence of the Si-V defect center Figure 1. Emissions from the Screening of Small Yellow Melee for in natural colorless gem diamonds,” silicon-vacancy defect at 736.6 and Treatment and Synthetics Diamond and Related Materials, Vol. 736.9 nm were detected in this Diamond treatment and synthesis 17, No. 7–10, pp. 1335–1344) and has 0.40 ct type IaB natural diamond. have undergone significant develop- been detected in very few natural type ments in the last decade. During this IIa and IaAB diamonds over the past showed blue fluorescence with natural time, the trade has grown increasingly several years. diamond growth patterns. These gemo- concerned about the mixing of treated Recently, a 0.40 ct round brilliant diamond with D color and VS2 clarity (figure 1) was submitted to the Hong Figure 2. The emission peaks at 736.6 and 736.9 nm from the [Si-V]– defect Kong laboratory for grading service. It are shown in the type IaB diamond’s photoluminescence spectrum. Iden- was identified as a pure type IaB natu- tical peak positions are detected in CVD synthetic diamonds. ral diamond. Infrared absorption spec- troscopy showed low concentrations PHOTOLUMINESCENCE SPECTRA –1 of the hydrogen peak (3107 cm ) and 736.9 N impurities in the B aggregates. Photo luminescence spectra at liquid- 736.6 nitrogen temperature with 514 nm laser excitation revealed [Si-V]– doublet emissions at 736.6 and 736.9 nm (fig- ure 2), while 457 nm laser excitation Natural type IaB diamond revealed the H3 (503.2 nm) emission. DiamondView fluorescence images INTENSITY Editors’ note: All items were written by staff members of GIA laboratories. CVD synthetic type IIa diamond GEMS & GEMOLOGY, Vol. 50, No. 4, pp. 293–301. 735.0 735.5 736.0 736.5 737.0 737.5 738.0 738.5 © 2014 Gemological Institute of America WAVELENGTH (nm) LAB NOTES GEMS & GEMOLOGY WINTER 2014 293 Figure 3. This group of 359 in- tensely colored round yellow dia- monds (0.02–0.03 ct) was screened by GIA’s New York labo- ratory. Among these, the majority were natural (left), 14 were HPHT synthetics (middle), and one was HPHT-treated natural diamond (right). They all displayed uni- form appearance and could not be distinguished visually. and/or synthetic diamonds with natu- and treated diamonds found in this when determining diamond origin. ral melee-sized goods. Because it is parcel, we analyzed an additional par- This study indicates that melee di- often not feasible to test every small cel containing 525 samples, with amonds in the marketplace are being diamond in a parcel, many of these similar results. One of these was contaminated by synthetic and treated products could be traded without being HPHT-processed, while 10 displayed diamonds. Screening analysis by gem tested individually by a gem lab. GIA’s the spectral, microscopic, and fluo- labs is essential to ensuring the correct New York laboratory recently tested rescence characteristics typical of identification. An efficient and reli- two large groups of melee yellow dia- HPHT synthetics; we concluded that able screening can be performed using monds submitted for screening of the remaining 514 were natural and infrared absorption spectroscopy, Dia- treatments and synthetics. The results untreated. Typical DiamondView mondView fluorescence imagery, and will likely have profound implications charac teristics of HPHT synthetics optical microscopy. From our analysis for how melee diamonds are handled were observed, but most of the HPHT of 883 melee samples in all, we con- in the trade. synthetics showed fluorescence pat- cluded that 857 were natural diamond A parcel of 359 round diamonds be- terns similar to those of natural dia- (97.1%), 24 were HPHT synthetic di- tween 0.02 and 0.03 ct was submitted monds (figure 4). This observation amond (2.7%), and two were HPHT- for identification. They showed uni- emphasizes the importance of in- processed natural diamonds (0.2%). form appearance, intense yellow color, frared absorption spectroscopy, in ad- Wuyi Wang, Martha Altobelli, and good clarity. Based on spectro- dition to DiamondView imaging, Caitlin Dieck, and Rachel Sheppard scopic analysis, each sample’s color was attributed to trace concentrations of isolated nitrogen. Gemological ob- Figure 4. These DiamondView fluorescence images show (A) growth sec- servations, infrared absorption spec- tor patterns typical of HPHT synthetic diamonds, (B) HPHT treatment, troscopy, and DiamondView analysis and (C and D) uncharacteristic HPHT synthetic patterns. confirmed that 344 of them were nat- ural diamonds, 14 were grown by HPHT (high-pressure, high-tempera- ture) synthesis, and one was an HPHT- treated natural diamond (figure 3). Of the 14 synthetic diamonds, eight were dominated by A-aggregate form nitrogen with trace isolated ni- trogen, while the other six showed negligible amounts. All had wide- spread pinpoint inclusions, a feature A B typical of HPHT synthetic diamonds. Characteristic growth features of HPHT synthesis were confirmed using a DiamondView fluorescence instrument on six of them. The syn- thetic diamonds with a high concen- tration of A-aggregate form nitrogen were produced at much higher tem- peratures, indicating more than one producer. To gain perspective on the surpris- C D ingly high prevalence of synthetic 294 LAB NOTES GEMS & GEMOLOGY WINTER 2014 VIS-NIR ABSORPTION SPECTRUM Very Large Irradiated Yellow 415.2 Artificial irradiation, with or without 1.2 (N3) annealing, has been used to improve the color of natural diamonds for sev- 1.0 eral decades. This technique is usu- 496.0 503.2 (H4) (H3) ally applied to brownish or light yellow diamonds of relatively small 0.8 size. The New York lab recently ABSORBANCE tested a very large yellow diamond 0.6 that had been artificially irradiated. 595 This emerald-cut stone weighed 22.27 ct and was color graded as Fancy 0.4 Vivid yellow (figure 5). Faint color con- centration was observed along the 400 450 500 550 600 650 700 750 800 culet. The diamond showed medium WAVELENGTH (nm) yellow-green, slightly chalky fluores- cence to long-wave UV and weak or- Figure 6. The irradiated yellow diamond’s Vis-NIR absorption spectrum, ange fluorescence to short-wave UV. collected at liquid-nitrogen temperature, showed strong absorption from No obvious internal features were ob- the H3 and H4 defects, which were artificially introduced to enhance the served. Its infrared absorption spec- yellow color. trum showed high concentrations of nitrogen and weak absorptions from defects H1b (4935 cm–1) and H1c enhancing its yellow color. This Fancy Kiefert et al., “Cultured pearls from (5165 cm–1). A weak hydrogen-related Vivid yellow color would have been the Gulf of California, Mexico,” absorption at 3107 cm–1 was also de- much lighter and less saturated before Spring 2004 G&G, pp. 26–38) are ex- tected. In the Vis-NIR region, the the treatment. amined in GIA laboratories from time absorp tion spectrum collected at Artificially irradiated diamonds of to time. But a recent submission to liquid-nitrogen temperature showed this size and attractive color are rarely the New York lab of 10 loose pearls, strong absorptions from the N3, H4, examined in gem laboratories. ranging from 8.17 × 7.65 × 3.89 mm to H3, and 595 nm optical centers (figure Wuyi Wang 17.85 × 11.16 × 6.80 mm (figure 7), 6). These spectroscopic and gemologi- proved interesting owing to their cal features demonstrated that the di- shapes and internal growth features. amond had been artificially irradiated All the pearls had a “grapelike” clus- and annealed to introduce additional Natural PEARL Aggregates from ter appearance, as if multiple smaller absorptions from the H3/H4 defects, Pteria Mollusks pearls had combined into aggre- Both natural and cultured pearls of gates. Their colors ranged from dark Pteria-species mollusks from the Gulf brown and purple to gray, with strong Figure 5. This 22.27 ct Fancy of California, Mexico (M. Cariño and orient consisting of mainly bluish and Vivid yellow diamond was iden- M. Monteforte, “History of pearling pinkish overtones. tified as artificially irradiated in La Paz Bay, South Baja California,” Microscopic examination revealed and annealed. Treated diamonds Summer 1995 G&G, pp. 88–104; L. that the pearls formed with continu- of this size and attractively satu- rated color are rare. Figure 7. These 10 loose natural pearl aggregates from the Pteria species ranged from dark brown and purple to gray. LAB NOTES GEMS & GEMOLOGY WINTER 2014 295 Figure 8. Microradiograph images of the pearls revealed multiple growth units with central conchiolin-rich cores in each unit. ous nacre layers. Microradiography coast, which further supports the flattened gas bubbles and voids that further demonstrated that their inter- identity of these unique pieces. are obvious under magnification (S.F. nal structures were composed of mul- Sally Chan and Yixin (Jessie) Zhou McClure et al., “Identification and tiple small pearls related to their durability of lead glass–filled rubies, growth (figure 8).
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