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Fall 1996 Gems & Gemology Gem News

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Fall 1996 Gems & Gemology Gem News GEM Editors Mary L. Johnson and John I. Koivula Contributing Editors Dino DeGhionno, Robert C. Kammerling, Shane F, McClure, GIA GTL, Santa Monica, California Henry A. Hanni, SSEF, Basel, Switzerland Karl Schmetzer, Petershausen, Germany DIAMONDS plus other types, whereas type Da near-colorless natural Chatham synthetic "white" diamonds at JCK show. diamonds are relatively rare. Three years ago, Tom Chatham of Chatham Created Energy-dispersive X-ray fluorescence (EDXRF] spec- Gems, San Francisco, California, announced that he troscopy disclosed both iron (Fe)and germanium (Gel in would be marketing "white" synthetic diamonds from all four crystals. GIA Research had not previously detect- Russia for jewelry use. Although it took longer than he ed germanium in any diamond, natural or synthetic. originally anticipated, Mr. Chatham offered for sale a They suspect that Ge is being added to the growth envi- number of near-colorless synthetic diamond crystals at ronment of these crystals to prevent nitrogen from incor- the June 1996 JCK show in Las Vegas, Nevada. porating into the crystal structure of the synthetic dia- Mr. Chatham offered GLA Research a brief opportu- monds, as the nitrogen would color them yellow. nity to examine about 100 of these synthetic diamond Although all of these properties indicated that these crystals (which ranged from about 10 points to almost a samples were synthetics, conclusive proof was provided carat) before the show. Most were of small size and had most readily by the metallic inclusions, which were eye- too many inclusions for faceting. Research Associate visible in all four samples, and were easily identified with Sam Muhlmeister and Research Gemologist Shane Elen magnification. All four proved to be magnetic. The pres- focused their testing on four crystals of slightly better- ence of Ge in these samples also provides proof of synthe- than-average quality. These weighed between 0.41 and sis, but trace-element determination requires the use of 0.51 ct, and were cubo-octahedral in crystal habit. All equipment beyond the reach of the average gemologist. four had eye-visible metallic inclusions (figure 11, one Mr. Muhlmeister and Mr. Elen cautioned that this prelim- had a typical white "cloud," one had relatively large inary study involved only four crystals; the properties of stepped cavities (which resembled the hopper-growth other Chatham synthetic "white" diamonds may differ. cavities in, for instance, salt crystals, but are unusual in natural diamonds), and two had surface structures that looked like trigons. All four crystals were attracted to a Figure 1. Chatham white" synthetic diamonds strong magnet. were recently offered for sale at the Las Vegas The crystals were inert to long-wave ultraviolet racli- ICK Show. Note the numerous metallic inclu- ation, but fluoresced a very faint yellow or orange to sions-characteristic of synthetic diamonds-in short-wave UV. The UV fluorescence was typical, both in this 0.51 ct example. Photo by Shane Elen. color and intensity, of that seen thus far in near-colorless synthetic diamonds; it is rare for a natural diamond to have a stronger fluorescence to short-wave than long- wave UV. However, no cross-shaped or octagonal pattern was visible in the UV fluorescence reaction-unlike the patterns reported previously for some synthetic diamonds (see, e.g., J. E. Shigley et al., "A Chart for the Separation of Natural and Synthetic Diamonds," Winter 1995, pp. 256-2641, The crystals phosphoresced blue for at least one minute after exposure to short-wave UV;phosphores- cence was much fainter in one than in the other three. On the basis of their infrared spectra, we determined that these four samples were all type IIa (essentially nitrogen-free) synthetic diamonds. Near-colorless syn- thetic diamonds are typically type Da or mixed type Da 214 Gem News GEMS & GEMOLOGY Fall 1996 cut from similar material. The gemological properties (see below) confirmed that this cushion-shaped modified brilliant cut was a natural chrysoberyl. We were soine- what surprised that there was no perceptible change of color (alexandrite effect) between incandescent and fluo- rescent light in a stone of such saturated green. With nlagnification, we noted a small brown-red crystal in the pavilion and growth zoning. EDXRF spectroscopy showed that the stone contained Al, Fe, V, Ti, Ga, and Sn. We had seen synthetic nonphenomenal green chrysoberyls colored by vanadium, but we had not previ- ously examined natural vanadium-bearing chrysoberyls. For comparison, we borrowed from Malhotra Inc., New York City, four faceted nonphenomenal green chrysoberyls (figure 3) that reportedly had been mined in Tanzania. Three had properties similar to the chrysoberyl in question. These included: pleochroism-trichroic colors of bluish green to blue-green/yellow-green to greenlnear- colorless to yellow; optic character-biaxial positive; color- filter reaction-none to faint pink; refractive indice* 1.742 and 1.750-1.75 1; birefringenceÑ0.008-0.009 specific Figure 2. This 2.14 ct diamond (9.40x 8.46 x 5.26 mm) gravity-3.713.72; lun~inescenceto UV radiation-inert is an example of the "Buddha Cut." Photo by Maha to faint orange (long wave] and inert to faint yellow (short DeMaggio. wave); no lun~inescenceto visible radiation; absorption spectrum in the desk-model spectroscopeÑ440-44 nm band; infrared spectrum-strong features at 3225 and Diamond novelty-cut as a seated Buddha. New forms of 2975 cm-1, weaker features at 3140, 2850, and 2650 cm-1; fashioned diamonds are designed either for novel light- W-visible spectrum-bands at 415 and 608 nni, peaks at transmission and weight-retention properties, or to resem- 318, 366, 375, and 380 nm. Again, EDXRF spectroscopy ble other items. Examples of the former cuts include the revealed All Fe, V, Ga, and Sn. The fourth stone (1.47 ct), 'Flanders Brilliant" (Gem News, Summer 1993, pp. which was bluish green, showed additional features typi- 130-131) and the "Context" and "Spirit Sun" cuts (Gem cally associated with chromium (including a red reaction News, Spring 1995, pp. 59-60). Examples of the latter to the Chelsea color filter; moderate red luminescence to include diamonds cut to resemble dice (Lab Notes, Fall 1985, p. 172)and letters of the alphabet (Lab Notes, Spring 1986, p. 47). We recently had the opportunity to examine Figure 3. These four vanadium-bearing a new cut, the "Buddha Cut" (figure 2), which is being chrysoberyls (1.47-13.52 ct) are reportedly from marketed in the United States by J. Kleinhaus and Sons Tanzania. The smallest contains chromium as of New York City. The distribution of facets was reini- well as vanadium, but none shows change of niscent of triangular modified brilliants, with 33 crown color. Photo by Maha DeMaggio. facets and 21 pavilion, facets (no culet). The girdle was also faceted; two GIA Gem Trade Laboratory (GTL] graders assessed the girdle as thick to extremely thick. Some consumers wear "Buddha Cut" diamonds in necklaces, while others treat them as objets d'art, according to a J. Kleinhaus spokesperson. (The unmount- ed sample we examined did "sit up" by itself.] Because of the potential religious implications, the cutter reportedly has strict criteria governing cut symmetry and what con- stitutes a diamond appropriate for this cut. For example, the "head" region should be free of.unsightly inclusions. COLORED STONES Noiiphenomenal vanadium-bearing chrysoberyl. Attrac- tive examples of green chrysoberyl, lacking change of color, were seen at the February 1996 Tucson gem shows; some material was marketed as "mint" chrysoberyl. Six months earlier, the editors had examined a 3.48 ct stone Gem News GEMS & GEMOLOGY Fall 1996 birefringence was 0.028. Both stones had an S.G. of 3.30, were inert to both short- and long-wave UV radiation, and had very dark spectra as seen with a handheld spec- troscope: The cut-off was at about 520 nm, with "chrome bands" at 630, 660, and 690 nm. Both stones contained scattered small crystals and "fingerprints," vis- ible with magnification but not particularly distinctive. EDXRF spectra of the two stones showed major Mg, Al, and Ca, with smaller amounts of Fe, Cr, Nil Ti, and Sr. "Rose"-colored plagioclase-corundum rock from Sri Lanka. In March 1995, contributing editor Henry Hanni Figure 4. Electron microprobe analyses revealed received two small "rose" pink translucent pebbles (fig- significant differences in chemical content ure 6) that each had one small polished face. These between this natural 11.14 ct bluish green vanadi- stones reportedly came from a locality near Pallebcdda, um chrysoberyl from the Tunduru area in southern Ratnapura district, Sri Lanka. The sender could not iden- Tanzania and the two synthetic chrysoberyls tify them on the basis of the R.I. (1.576)and S.G. (2.98) (right, 1.00 and 1.12 ct) produced in Russia. The values that he determined. He also noted weak chromi- natural stone is courtesy of W.Spaltenstein, um lines in the absorption spectrum. Bangkok; photo courtesy of SSEF. At the SSEF in Basel, microscopic observation of the surface in reflected light revealed a granular groundmass visible light; and a 590-665 nm absorption-and 670 nin (90%)with occasional interspersed aggregates of idiomor- emission line-seen with the handheld spectroscope, as phic crystals, which ranged from 20 to 70 microns (10%). well as Cr in the EDXRF spectrum); however, there still The pink crystals had a hardness greater than that of the was no perceptible change-of-color. groundmass, as seen by their relatively higher relief. Contributing editor Henry A. Hanni had the oppor- EDXRF analysis of the entire pebble indicated that Si, Al, tunity to examine an 11.14 ct "intense" bluish green and Ca were the main constituents, with Cr, Fe, and Sr chrysoberyl (figure 4) that was reportedly from Tunduru, present as trace elements.
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