gems, tourmaline, Kenya

Tourmaline from a recent find in Kenya, referred to as “savannah” or “golden” tourmaline, has attracted attention for its distinctive coloration. Gemologist Yianni Melas submitted two rough samples for examination: a 4.50 ct transparent brownish orangy yellow piece with green areas, and a 3.06 ct crystal section showing a brownish orangy yellow core with a green rim. The broken crystal section displayed only prism faces, lacking striations parallel to the c-axis. Gemological properties recorded include pleochroism from brownish orange to yellow in the yellow regions and dark green to light yellowish green in the green areas, uniaxial optic character, refractive index of 1.630–1.650 with a birefringence of 0.020 (on the green rim), and specific gravity of 3.05. Under long-wave UV, the stones were inert, but showed moderately chalky yellow fluorescence under short-wave UV, stronger in the green rim. Spectroscopic analysis showed general absorption to ~480 nm, with additional bands between 590–630 nm and lines at 650 and 680 nm in the green rim. Inclusions observed included anhedral crystals, needles or tubes, two-phase inclusions, and hollow tubes. EDXRF spectroscopy revealed Mg, Al, Si, K, Ca, Ti, Fe, Zn, Ga, and Sr in both samples, with one showing a small Mn peak. Chromium was detected in the green rim, consistent with the observed color-filter and absorption data, indicating Cr as the chromophore for the green. Reported sources for this material include the Kasigau Mine in Tsavo West National Park, the Mangari area in the same park, and M’gama Ridge south of the Taita Hills in southeastern Kenya.