Color-Change Garnets from Tanzania
At the Riverpark Inn (Pueblo) show, Todd Wacks (Tucson Todd’s Gems,Tucson and Vista, California) and Jason Doubrava (Jason Doubrava Gems & Minerals, Poway, California) showed us interesting color-change or color-shift garnets from Tanzania (figure 1). According to them, these gems were mined in Morogoro, Tanzania, back in 1988, and documented the same year (see C.M. Stockton, “Pastel pyropes,” Summer 1988 G&G, pp. 104–106). The material resembled rhodolite rough and contained fine needle-like inclusions—most likely rutile.
The gems were pinkish purple in daylight and showed a color change from intense pink in warm incandescent light to purple—almost like a fine amethyst—in cool LED light. Wacks said the larger stones display the most pronounced color change. Very similar color change phenomena have been reported for other purple “pastel pyropes” from Tanzania, Sri Lanka, and Madagascar.
The rough had been stuck in a safe deposit box for years, he said, because most potential buyers assumed it was rhodolite, saw the inclusions, and lost interest. A consortium including Doubrava, Wacks, and well-known gem artist Meg Berry recently acquired the 2–4 kg of rough, cut a few pieces, and discovered the color change. Wacks has been promoting the gem since then.
The group sent samples to Dr. George Rossman (California Institute of Technology, Pasadena), San Diego gemologist Kirk Feral, and GIA. According to Wacks, the gems are approximately 80% pyrope, 10% spessartine, and 10% almandine, with an RI of 1.736. (This material will be the subject of a more detailed paper in the Winter 2015 G&G.)