Fall 2011

Featured Gem News International Entry: Color-change garnet from Nandagala, Tanzania

Mark Saul (Swala Gem Traders, Arusha, Tanzania) recently informed GIA’s Bangkok lab about a new deposit of color-change garnet near Nandagala village, Lindi Province, southern Tanzania. According to Mr. Saul, the stones were believed to be alexandrite when first discovered in January 2011, resulting in a minor gem rush and several dealers overpaying for rough. The garnet comes from two diggings: A primary deposit where small (typically <0.2 g) dark-colored stones are found, and a nearby alluvial deposit that produces somewhat lighter pieces up to ~2 g. One large (>5 g) clean stone from the alluvial deposit was faceted into the 11.81 ct gem in figure 1. The alluvial deposit also produces clean but nonphenomenal brownish orange garnets, some of which exceed 20 g.

The five rough samples in figure 1 were purchased by GIA, and four were polished with parallel windows and examined for this report. Their color change was distinct, showing greenish blue in fluorescent light and purplish red or red in incandescent light. They had RI and SG values of 1.762 and 3.89, which are typical for color-change pyrope-spessartine (e.g., D. V. Manson and C. M. Stockton, “Pyrope-spessartine garnets with unusual color behavior,” Winter 1984 G&G, pp. 200–207, http://dx.doi.org/10.5741/GEMS.20.4.200). They were inert to long- and short-wave UV radiation. With the desk-model spectroscope, the darkest sample showed a band at ~400 nm and a broad absorption at ~570 nm; the latter feature was less obvious for lighter stones with a weaker color change. The inclusion scene (figure 2) was dominated by needles, zircon-like crystals, partially healed fissures, and negative crystals. We also saw some black opaque, reddish brown, and elongated greenish crystals. Unfortunately, the inclusions were too deep in the samples to be identified with Raman spectroscopy.

Chemical composition was measured using EDXRF spectroscopy and a Thermo X Series II LA-ICP-MS.  The garnets had an average composition of Pyp₅₁Sps₄₀-Grs_3.5Alm₃Gol₂Uva_0.5 (with 7300 ppm V and 1100 ppm Cr). This was very similar to that reported for color-change garnets from Bekily, Madagascar (K. Schmetzer and H. J. Bernhardt, “Garnets from Madagascar with a color change of blue green to purple,” Winter 1999 G&G, pp. 196–201, http://dx.doi.org/10.5741/GEMS.35.4.196).

UV-Vis absorption spectra were collected using a PerkinElmer Lambda 950 spectrometer. A strong absorption band at ~570 nm (due to V³⁺ and Cr³⁺) dominated the spectra of all samples (e.g., figure 3). This band created two transmission windows, in the blue-green (~480 nm) and red (700 nm) regions, which are responsible for the color change. The darkest sample showed a cutoff at ~440 nm, while the lighter stones had a cutoff at ~310 nm and also showed absorptions due to Mn²⁺ (408, 423, and 489 nm), Fe³⁺ (432 nm), and Fe²⁺ (463 nm; see P. G. Manning, “The optical absorption spectra of the garnets almandine-pyrope, pyrope and spessartine and some structural interpretations of mineralogical significance,” Canadian Mineralogist, Vol. 9, 1967, pp. 237–251).

This new Tanzanian deposit joins Kenya and Madagascar as another source of color-change garnet in the East African region.

Vincent Pardieu, Kamolwan Thirangoon, and Sudarat Saeseaw
GIA, Bangkok