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IDENTIFICATION of HPHT-TREATED YELLOW to GREEN DIAMONDS by Ilene M

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IDENTIFICATION of HPHT-TREATED YELLOW to GREEN DIAMONDS by Ilene M IDENTIFICATION OF HPHT-TREATED YELLOW TO GREEN DIAMONDS By Ilene M. Reinitz, Peter R. Buerki, James E. Shigley, Shane F. McClure, and Thomas M. Moses Examination of recently introduced greenish everal companies are now treating brown diamonds yellow to yellowish green HPHT-treated dia- with high pressure and high temperature (HPHT) to monds from three companies revealed several transform their color to greenish yellow, yellowish identifying characteristics. Gemological prop- S green, yellow, or brownish yellow (figure 1). These include erties include a highly saturated body color, the General Electric (GE) Company, Novatek, and unidenti- well-defined brown to yellow octahedral fied organizations in both Russia and Sweden. Both GE and graining, moderate to strong green “transmis- Novatek are concentrating their efforts on producing colors sion” luminescence to visible light (associated with the graining), and visual evidence of with an obvious green component (Templeman, 2000; heating. Most of the treated diamonds, includ- Federman, 2000; Anthony et al., 2000). In contrast, the ing some from each of the three sources, Swedish manufacturer apparently is trying to produce colors exhibit chalky greenish yellow to yellow-green similar to those of natural yellow diamonds (J. Menzies, fluorescence to UV radiation (long- and short- pers. comm., 2000). Moses and Reinitz (1999) briefly wave). Distinctive features seen with a hand- described some of the treated diamonds from these various held spectroscope include a 415 nm line, a manufacturers. The present article reports the gemological strong band from about 480 to 500 nm, a and spectroscopic properties of a large number of diamonds strong line at 503 nm, and emission lines at treated in this fashion, and discusses how the gemological 505 and 515 nm. IR spectra reveal that they properties compare to those of the rarest and most desirable are type Ia diamonds. Near-IR spectra show a of the stones that they resemble: natural-color greenish yel- peak at 985 nm, an indicator of high-tempera- ture exposure. A small number of these HPHT- low to yellow-green diamonds. treated diamonds are yellow to brownish yel- low; they do not show any green transmission, BACKGROUND but other properties identify them as treated. An article by Shigley et al. (1993) on gem-quality synthetic yellow diamonds from Novosibirsk, Russia, described three ABOUT THE AUTHORS greenish yellow to yellow synthetic diamonds that had been Dr. Reinitz is manager of Research and heat treated at high pressure to alter their color. This treat- Development, and Mr. Moses is vice president ment was done in the same apparatus in which the dia- of Research and Identification, at the GIA Gem monds were synthesized. Although the gemological and Trade Laboratory, New York. Dr. Buerki is a research scientist, and Dr. Shigley is director, at spectral properties of those synthetics are rather different GIA Research, Carlsbad. Mr. McClure is director from the treated-color natural diamonds reported on here, of Identification Services at the GIA Gem Trade we believe that article was the first report in the gemologi- Laboratory, Carlsbad. cal literature on HPHT treatment to alter diamond color. Please see acknowledgments at the end of the From fall 1996 through winter 1997, approximately 50 article. natural diamonds of unusual greenish yellow to yellow- Gems & Gemology, Vol. 36, No. 2, pp. 128–137 green color were submitted to the GIA Gem Trade © 2000 Gemological Institute of America Laboratory for identification reports (Reinitz and Moses, 128 Yellow to Green HPHT Diamonds GEMS & GEMOLOGY Summer 2000 Figure 1. These diamonds have all been treated at high pressure and high temperature (HPHT). Clockwise from top right: 1.01 and 0.52 ct diamonds treated by GE; yellow diamonds (0.23–1.01 ct) treated by an unknown source in Sweden; rough dia- monds after treatment by Novatek, 0.88–1.68 ct. GE and Novatek photos by Maha Tannous; photo of Swedish stones by Elizabeth Schrader. 1997). These diamonds displayed gemological and ments (T. Anthony and D. Hall, pers. comms., 2000). spectroscopic properties—saturated color, strong Using the same types of apparatus as are used to syn- graining, burn marks and other visual evidence sug- thesize single-crystal diamonds, they place natural gestive of heating, and certain bands in the absorp- diamonds under a high confining pressure of about 6 tion spectra—that, in light of the current study, sug- Gigapascals (GPa, equal to 60 kilobars). While the gest that some of them may have been treated using stones are at high pressure, they expose them to high HPHT technology very similar to the current tech- temperatures, up to 2100°C, for short periods (i.e., nique. We do not know where those diamonds were less than 30 minutes, in some cases much less). treated. However, Van Bockstael (1998) described These conditions are similar to those used by GE to diamonds with similar colors and gemological char- decolorize diamonds (Anthony and Casey, 1999), but acteristics, and stated that they had undergone the starting materials are nitrogen-bearing type Ia HPHT treatment in Russia. Henn and Milisenda brown diamonds, rather than nominally nitrogen- (1999) reported on similar material, as did De free type IIa diamonds. As with the GE POL decol- Weerdt and Van Royen (2000). Buerki et al. (1999) orization process, however, polished diamonds must wrote a technical report on the spectra and features be repolished to remove surface damage after HPHT of both natural and treated type Ia yellow diamonds exposure. Although the treated diamonds produced with green luminescence, and hypothesized as to from these two sources are generally similar in what kinds of treatment might yield the features appearance and gemological properties, this does not observed. Recently, Collins et al. (2000) reported the mean that these companies are using identical infrared- and visible-range spectroscopic features of HPHT equipment and treatment conditions. HPHT-treated diamonds, and discussed how the However, it is not our intention to compare in detail treatment conditions affect the color centers in type the specific products of the various manufacturers. Ia diamond. Rather, this article will focus on those gemological Both GE and Novatek have been open to dis- characteristics that will facilitate identification of cussing details of how they perform these treat- this kind of treated diamond in general. Yellow to Green HPHT Diamonds GEMS & GEMOLOGY Summer 2000 129 TABLE 1. Properties of HPHT-treated diamonds from GE, Novatek, and an undisclosed source in Sweden. Property Major group (57)a Minor group (6) Color Greenish yellow to Yellow to brownish yellow-green (36); yellow (6) yellow-green to yellowish green (21) Color zoning Brown to yellow Brown graining (4); (seen in graining (41); none (16) none (2) diffused light) Microscopic Etched naturals or Etched naturals or features feathers, or tension feathers (5); no fractures, often containing naturals or fractures (1) graphite (32); naturals without a frosted appearance (2); no naturals or fractures (23) Figure 2. Brown to yellow planar graining was UV fluorescence found in most of the treated diamonds examined Long-wave Greenish yellow to Green to greenish in this study. Photomicrograph by James E. yellowish green, strong in yellow (3); blue plus Shigley; magnified 10×. most cases, with a chalky yellow (1); chalky appearance (39); blue, appearance (4); none (2) plus yellow or greenish yellow, with a chalky MATERIALS AND METHODS appearance (17); none (1) Short-wave Greenish yellow to Green to greenish For this study, we examined 63 diamonds that were yellowish green (53); yellow (3); yellowish known to have been HPHT treated: 41 faceted (round yellow (4); chalky appear- green + blue (3); ance (40—includes the chalky appearance (4) brilliants and fancy shapes) and 22 rough. Thirty-one 4 that showed yellow) of the samples were from GE, 25 were from Novatek Luminescence Moderate to strong None (4); weak blue (including all 22 rough stones), and seven were from excited by green (56); moderate plus green (2) visible light green plus weak blue(1) the Swedish manufacturer. Seven of the rough dia- ("transmission") monds were examined before and after they were Spectroscope Strong 503 nm line (57); 415 nm line (1); no treated by Novatek. We polished small “windows” spectrum dark band from 480– 500 lines (5) on 11 of the rough samples (including two of the nm (41)—of these 21 show a 415 nm line before-and-after samples) to facilitate examination of and 20 show emission their internal features and recording of spectra. All lines at 505 and 515 nm samples were examined with a binocular gemologi- UV-Vis-NIR Weak peak at 415 nm, Sharp rise in cal microscope and various illumination techniques, absorption strong to very strong peak absorption toward spectra at 503 nm, and weak to shorter wavelengths a long-wave (366 nm) and short-wave (254 nm) strong peak at 985 nm with no sharp peaks Ultraviolet Products UV lamp unit, and both a Beck (45); weak broad band (4); or, sharp rise in prism and a Discan digital-scanning diffraction-grat- centered at 550 nm (29); absorption toward weak peak at 535 nm shorter wavelengths ing spectroscope. Two Hitachi U-4000 series spec- (21); weak peak at 637 with weak peak at 985 trophotometers were used to record absorption spec- nm (14); or weak peaks nm (1); weak peaks at 415 and 503 nm, at 415 and 503 nm, tra at liquid-nitrogen temperature over the range of weak to moderate peak weak to moderate 250–1000 nm. Infrared spectra were recorded with a at 985 nm, and rising peak at 985 nm, and Nicolet 760 Fourier-transform infrared (FTIR) spec- absorption toward rising absorption shorter wavelengths (9) toward shorter -1 trometer over the range 400–25,000 cm , or a wavelengths (1) Nicolet 550 FTIR spectrometer over the range 400- Mid-infrared IaB >A (26) IaA >B (2) 11,000 cm-1.
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