Fall 2000 Gems & Gemology - Gemfrance

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Figure 9. This 0.69 ct fresnoite was our first encounter with this mineral as a fashioned gem. Courtesy of C. D. (Dee) Parsons; photo by Maha Tannous. We first sought to confirm that the stone was fresnoite. We recorded refractive indices of 1.765–1.773, yielding a birefringence of 0.008. A uniaxial optic figure, as would be expected from a tetragonal mineral, was clearly visible in cross-polarized light through the table facet. The two dichroic colors observed were yellow and near-colorless. The stone was inert to long-wave UV radiation, but it fluoresced a strong, slightly chalky whitish yellow to short-wave UV. Examination with a Beck prism spectroscope revealed a weak 447 nm absorption line. The specific gravity, an average of three sets of hydrostatic weighings, was 4.60. Since all of the above properties matched, within acceptable tolerance, those previously recorded in the mineralogical literature for fresnoite (see, e.g., Gaines et al., Dana’s New Mineralogy, 1997, p. 1145), we concluded that this faceted gem was in fact fresnoite. To characterize the material in detail, we performed advanced testing. Energy-dispersive X-ray fluorescence (EDXRF) qualitative chemical analysis by GIA Gem Trade Lab research associate Sam Muhlmeister revealed abundant barium and silicon, as expected. However, it was difficult to determine the presence of titanium by this technique, because of interference from the strong barium peaks. With the permission of Mr. Parsons, an X-ray powder diffraction pattern was obtained by Gem News editor Dino DeGhionno. Comparison of the powder pattern with a standard materials database confirmed the earlier gemological identification as fresnoite. Shane Elen, of GIA Research, recorded a Raman spectrum so that it could be added to our Raman database. Only a few weeks after this initial examination, and entirely independent of the earlier study, we received more fresnoite for examination, a 0.20 ct shield-shaped mixed cut and an 8.24-mm-long crystal that weighed 1.86 ct (figures 10 and 11). These examples were provided by Michael Gray of Graystone Enterprises in Missoula, Figure 10. This 0.20 ct faceted fresnoite and 1.86 ct crystal are reportedly from the Junnila mine in San Benito County, California. Courtesy of Michael Gray; photo by Maha Tannous. Montana, who stated that they came from the Junnila mine in San Benito County, California, just a few miles from the famous Benitoite Gem mine (see S. Kleine, “The great fresnoite discovery of 1998,” Rock and Gem, Vol. 29, No. 3, 1999, pp. 52–59). With the data obtained from the earlier stone, it was relatively easy to identify both of these samples as fresnoite. The only discrepancy was the 4.51 S.G. of the cut stone, compared to the 4.60 previously obtained. However, specific gravity determination is less certain for such a small stone. Because fresnoite also has a synthetic counterpart grown by the Czochralski pulling process (see, e.g., Figure 11. The faceted fresnoites contained numerous partially healed fractures, which are useful in separating them from nearly flawless Czochralskipulled synthetics. Photomicrograph by John I. Koivula; magnified 15×. Gem News GEMS & GEMOLOGY Fall 2000 265
U. Henn, “Synthetischer Fresnoit,” Gemmologie: Zeitschrift der Deutschen Gemmologischen Gesellschaft, Vol. 48, No. 4, 1999, pp. 232–233), it is important to be able to separate this rare natural collector’s gem from its synthetic equivalent. By design, Czochralski pulling tends to result in nearly flawless crystals. During his examination of a faceted 6.11 ct Czochralski-grown synthetic fresnoite, the only inclusion noted by Henn (1999) was a small spherical gas bubble visible at 40× magnification. This is in sharp contrast to the numerous “fingerprint” fluid inclusions along partially healed fractures (figure 11) that we observed in the faceted natural fresnoites. The other gemological properties were essentially identical. Therefore, observation of inclusions serves as an important means to separate natural fresnoite from its synthetic counterpart. Figure 12. Some superb gem tourmaline crystals have been recovered from pegmatites in Brazil. This specimen (10 cm tall) of tourmaline on quartz and cleavelandite is from the Santa Rosa area in Minas Gerais. Courtesy of Wayne Thompson; photo by Jeff Scovil. Gemological Presentations at the 31st International Geological Congress. From August 6 to 17, more than 4,000 participants from 103 countries convened in Rio de Janeiro for the 31st International Geological Congress, which featured three sessions on gems. Abstracts of all 6,179 presentations were supplied to Congress participants in searchable format on CD-ROM (some of these were submitted and accepted, but were not actually presented at the meeting). A searchable database containing many of the abstracts is also available on the Congress Web site (www.31igc.org) until 2002. Presentations relating to gemstones encompassed geology, localities, treatments, and gem identification; the following were attended by this contributor. Brazilian gemstones were highlighted in several talks. C. P. Pinto (CPRM-Serviço Geológico do Brasil, Belo Horizonte) and A. C. Pedrosa-Soares (Federal University of Mina Gerais, Belo Horizonte) provided a useful compilation of the geology of Brazilian gem deposits. Most are related to granitic pegmatites, hydrothermal veins, or geodes in basaltic lava flows. Emerald, aquamarine, tourmaline, topaz, chrysoberyl, alexandrite, amethyst, citrine, agate, opal, and morganite are the main gem materials being produced. R. Wegner (Federal University of Paraíba, Campina Grande) and co-authors provided updates on recent gem and mineral discoveries in the states of Paraíba and Rio Grande do Norte. These include gem-quality crystals of “golden” beryl (up to 7 cm long) and herderite (up to 12 cm long) from the Alto das Flechas pegmatite, color-zoned yellow-green-blue apatite (nearly 20 cm long) from the Alto Feio pegmatite, and cat’s-eye triplite from the Alto Serra Branca pegmatite. Wegner et al. also reviewed important Brazilian tourmaline and aquamarine deposits. Significant gem deposits are found in numerous pegmatites within two provinces: Oriental (or Eastern) in the states of Minas Gerais, Bahia, and Espírito Santo, and Northeastern in Ceará, Paraíba, and Rio Grande do Norte. In both provinces, gem tourmaline (figure 12) is found in highly evolved granitic pegmatites, whereas aquamarine typically forms in less-differentiated granitic pegmatites. M. V. B. Pinheiro (Federal University of Mina Gerais, Belo Horizonte) and co-authors used electron paramagnetic resonance, Mössbauer spectroscopy, and optical absorption to study natural and treated (irradiated and/or heated) Brazilian gem tourmalines. In pink elbaite, they confirmed the presence of Mn 2+ and noted that irradiation intensified the pink color, while heating to about 450°C decolorized pink and blue crystals and lightened green ones. N. L. E. Haralyi (IGCE-UNESP, Rio Claro) noted that white (not colorless) diamonds may show brownish red, orange, yellow, and greenish yellow “opalescent” colors; they also may have rather low specific gravities (e.g., 3.46 for one from Juina in northwest Mato Grosso State). Geologic investigations of numerous gem materials were presented. G. Harlow (American Museum of 266 Gem News GEMS & GEMOLOGY Fall 2000
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pg. 237 pg. 217 pg. 247 REGULAR FEA
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When a type IIa diamond is exposed
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Page 66 and 67: REFERENCES Arem J.E. (1987) Color E
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Fall 2000 Gems & Gemology - Gemfrance

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