corundum, Tanzania.12, gems

This note describes natural green vanadium-bearing chrysoberyl that lacks a perceptible alexandrite colour change, together with comparative observations on Russian synthetic material. Green chrysoberyls, some marketed as “mint” chrysoberyl, appeared at the February 1996 Tucson gem shows. Six months earlier, the editors had examined a saturated green, 3.48 ct cushion-shaped modified brilliant whose gemmological properties established its identity as natural chrysoberyl. Despite its strong colour, it showed no perceptible change between incandescent and fluorescent illumination. Microscopy revealed growth zoning and a small brownish red crystal in the pavilion, while energy-dispersive X-ray fluorescence spectroscopy detected aluminium, iron, vanadium, titanium, gallium and tin. Four additional faceted green chrysoberyls, weighing 1.47–13.52 ct and reportedly mined in Tanzania, provided comparative material. Three had properties similar to those of the initial stone: biaxial positive optic character, refractive indices of 1.742 and 1.750–1.751, birefringence of 0.008–0.009 and specific gravity of 3.71–3.72. Their trichroism ranged through bluish green or blue-green, yellow-green or green, and near-colourless or yellow. Ultraviolet fluorescence was absent to faint, and no luminescence occurred under visible light. Spectroscopic examination showed an absorption band at 440–445 nm, ultraviolet–visible absorption bands at 415 and 608 nm, and characteristic infrared features. Chemical analysis again detected vanadium with iron, gallium and tin. The fourth stone, a 1.47 ct bluish green specimen, also contained chromium and exhibited a red Chelsea filter reaction, moderate red luminescence under visible light and chromium-related spectral features, yet still showed no perceptible colour change. Separate electron microprobe analyses compared an intense bluish green, 11.14 ct natural chrysoberyl reportedly from Tunduru, Tanzania, with two Russian hydrothermal synthetic nonphenomenal chrysoberyls. The natural specimen contained 0.4 wt.% V₂O₃, 0.2 wt.% Fe₂O₃ and traces of chromium, tin and gallium. The synthetic specimens contained more vanadium and chromium, approximately 1.8 wt.% V₂O₃ and 0.2 wt.% Cr₂O₃, with no appreciable iron, tin or gallium. These observations document natural vanadium-bearing green chrysoberyl without alexandrite behaviour and identify compositional differences from the synthetic samples examined.