Feature Gems & Gemology, Winter 2009, Volume 45, No. 4

Chrysoprase and Prase Opal from Haneti, Central Tanzania

Commercial quantities of gem-quality chrysoprase and green prase opal (nickel-bearing chalcedony and common opal, respectively) have been recovered from altered serpentinite deposits near Haneti, Tanzania. Material studied for this report came from the largest mine, located on top of Iyobo Mountain, which has been actively exploited for two decades. Although somewhat similar in color and appearance, chrysoprase and prase opal can easily be distinguished from one another by differences in refractive index (~1.55 vs. ~1.45) and specific gravity (~2.60 vs. ~2.11). Chrysoprase makes up the vast majority of the output, and the mine shows good potential for continued production.