Summer 2007

Featured Gem News International: Pyrope-Almandine from Tanzania

John D. Dyer (Precious Gemstones Co., Edina, Minnesota) had some attractive orange-red to orangy red garnets that he marketed as "rose malaya" at the 2007 Tucson gem shows. This trade name was based on the identification of the material as pyropespessartine by independent gemologists. The garnets reportedly have been produced since mid-2005 from Tanzania’s Umba Valley.

Mr. Dyer loaned three samples and donated an additional 1.35 ct garnet to GIA for examination (see figure). The following properties were determined on all four stones by one of us (EAF): color—orange-red; RI—1.742; no birefringence; hydrostatic SG—3.80; Chelsea filter reaction—none; fluorescence—inert to long- and short-wave UV radiation; and absorption lines at 504, 520, and 573 nm visible with the desk-model spectroscope. There was no observable shift in color between daylight-equivalent and incandescent light sources. These properties are similar to those reported for pyrope-spessartine, but they are more consistent with pyrope-almandine, according to C. M. Stockton and D. V. Manson ("A proposed new classification for gem-quality garnets," Winter 1985 Gems & Gemology, pp. 205–218). Microscopic examination revealed no inclusions and only minor surface abrasions on the stones.

EDXRF spectroscopy of the 1.35 ct garnet showed major amounts of Si, Al, Mg, and Fe, as well as minor Ca and Mn. Electron-microprobe analysis of the other three samples at the University of New Orleans confirmed the identification as pyrope-almandine, yielding the following components: 71.1–73.1% pyrope, 18.9–21.7% almandine, 7.0–8.5% grossular, and 0.2–0.4% spessartine, along with traces of the andradite component.

East Africa is a common source for pyrope-almandine that typically ranges from reddish orange to red-purple, with the latter color referred to as rhodolite by the gem trade. The red-purple coloration of rhodolite is very different from the orange-red color of the pyrope-almandine examined for this report. Rhodolite also commonly contains abundant oriented needle-like rutile crystals (e.g., P. C. Zwaan, "Garnet, corundum, and other gem minerals from Umba, Tanzania," Scripta Geologica, Vol. 20, 1974, pp. 1–41), which were not seen in the pyrope-almandine garnets we studied.

East Africa is also known for producing pyrope-spessartine in the pink to red to orange to yellow-orange range, which has been marketed as malaya garnet (see Stockton and Manson, 1985; K. Schmetzer and H. Bank, "Garnets from Umba Valley, Tanzania—Members of the solid solution series pyrope-spessartine," Neues Jahrbuch für Mineralogie, Monatshefte, Vol. 8, 1981, pp. 349–354). A wide range of compositions have been reported for malaya garnets from East Africa, with most having a 30–55% spessartine component, but some having as low as 10–30% spessartine (Stockton and Manson, 1985).

The garnets examined for this report did not contain enough spessartine component to be called malaya garnets, and they are best referred to as pyrope-almandine since they lack the purple hue of rhodolite.

Troy Blodgett and Eric Fritz
GIA Laboratory Carlsbad

William B. Simmons and Alexander U. Falster
University of New Orleans
New Orleans, Louisiana