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| These spinels show the range of color of the new material being mined in the Mahenge region of Tanzania. Courtesy of Advanced Quality; photo © Jeff Scovil. |
At the GJX show, Menahem Sevdermish of Advanced Quality, Ramat Gan, Israel, had some attractive spinel from a new find in the Mahenge region of south-central Tanzania. His stock consisted of about 2,000 carats of faceted stones (ranging up to 3 ct) and 1 kg of rough. According to Mr. Sevdermish, the new spinel discovery occurred in August 2003, although ruby and pink-to-red spinel have been mined from this area for several years (see Summer 1993 Gem News, pp. 136–137). Several kilograms of rough spinel were recovered, ranging from pink to pink-orange to brownish red. Most notable are the pink-orange stones, with a color similar to that of some “padparadscha” sapphires. He also indicated that the material is mined from alluvial and eluvial deposits, and is not heated or subjected to any other forms of treatment. The rough typically consists of small octahedra or broken fragments that weigh 0.1–2 grams; less than 30% is facetable, and the remainder is bead- or cabochon-quality, in Mr. Sevdermish’s experience.
Some of the rough has made its way to Asian markets. According to Scott Davies of American-Thai Trading (Bangkok), some dealers in Bangkok have been marketing the faceted orange-pink stones as spinel from Vietnam’s Luc Yen region. Mr. Davies reported that the largest stone he has cut weighed 3.39 ct, and that stones containing eye-visible inclusions can make attractive rose cuts.
Mr. Sevdermish loaned GIA six Mahenge spinels (0.68–2.28 ct) that were representative of the range of color for this material: purplish pink, pink, orangy pink, red-orange, and orangy red (see figure). Gemological properties obtained by one of us (EPQ) showed refractive indices ranging from 1.710 to 1.712 and (hydrostatic) specific gravity values of 3.60–3.63. There was no birefringence or pleochroism, as expected for a singly refractive gem material, but all samples displayed weak anomalous double refraction when viewed between crossed polarizing filters. Long-wave UV radiation produced a red fluorescence, which ranged from weak to moderately strong for the various samples. With short-wave UV radiation, the orangy red to red-orange spinels were inert, but the three pink stones fluoresced very weak to weak orangy red. When examined with a desk-model spectroscope, all samples showed several absorption lines in the red region (typical of Cr3+) and a weak band from 520 to 590 nm. Microscopic examination revealed that all of the spinels contained stringers of particles, three of them had clouds throughout, two had fractures, and one had a “fingerprint.” The stones did not exhibit any transmission luminescence, as has been documented previously for pink Tanzanian spinel (see the Summer 1990 Lab Notes, pp. 156–157).
The properties of these stones are generally consistent with those listed for spinel by R. Webster (Gems, 5th ed., revised by P. Read, Butterworth-Heinemann, Oxford, England, 1994, pp. 142–145), although the R.I.’s recorded for these stones are just slightly lower.
Elizabeth P. Quinn and Brendan M. Laurs
Gemological Institute of America
Carlsbad, California
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