Vibrant Green Grossular Garnet “Transvaal Jade”
At the Pueblo show, Tom Schneider of TMS Gems (San Diego, California) offered “Transvaal jade” rough displaying an exceptionally vivid green color. Also known as “African jade” or “South African jade,” this is not a true jade (jadeite or nephrite). The stone is instead a grossular garnet whose trade name is derived from its massive habit and green color, and the type locality. Specimens of this material are typically more translucent and lighter in color (J. Frankel, “Uvarovite garnet and South African jade (hydrogrossular) from the Bushveld Complex, Transvaal,” American Mineralogist, Vol. 44, No. 5-6, 1959, pp. 565–591). Nested in a fine-grained chromite matrix, the opaque green portion of the stone in the figure above had a refractive index of 1.731 and was inert to both long-wave and short-wave ultraviolet radiation.
The sample was identified as grossular garnet using Raman spectroscopy and further classified as a grossular-andradite garnet using X-ray fluorescence chemical analysis and the naming convention proposed in 1995 (M.L. Johnson et al., “Gem-quality grossular-andradite: A new garnet from Mali,” Fall 1995 G&G, pp. 152–166). The chemical data show 80.83 mol.% grossular (Ca3Al2Si3O12), 10.50 mol.% uvarovite (Ca3Cr2Si3O12), 7.11 mol.% andradite (Ca3Fe2Si3O12), and trace amounts of other garnet. Garnets in the grossular to andradite range can show 4.64–20.91 wt.% Fe2O3 (Johnson et al., 1995) and typically have low chromium values (≤0.23 wt.% Cr2O3) (C.M. Stockton and D.V. Manson, “A proposed new classification for gem-quality garnets,” Winter 1985 G&G, pp. 205–218). Increased iron content will impart a yellow, orange, or brown color component, while the most intensely green stones owe their color to chromium and possibly vanadium (Johnson et al., 1995). The low iron content (2.49 wt.% Fe2O3) and elevated chromium content (3.50 wt.% Cr2O3) of this sample likely resulted in its remarkable coloration.
The chromium-rich nature of this particular specimen produced a beautiful vibrant green with a high-polish luster, making it an outstanding example of this material.
