Chrysoprase and Prase Opal from Haneti, Central Tanzania

GEMS & GEMOLOGY WINTER 2009 271 hrysoprase and prase opal are nickel-containing green varieties of chalcedony and nonplay-of-color (common) opal, respectively. Both have been used as gem materials for thousands of years. Chrysoprase was described as being the “most prized” type of chalcedony by Webster (1994, p. 233). Central Tanzania is an important contemporary source of both these gems. The Tanzanian materials were first briefly described by Gubelin (1975, pp. 76–78) and Schmetzer et al. (1976). These reports were followed by a characterization of the green prase opal by Koivula and Fryer (1984), and a more detailed study of the chrysoprase by Kinnunen and Malisa (1990). This article briefly describes the geology, mining, and gemological characteristics of high-quality Tanzanian chrysoprase and prase opal from the Iyobo Mountain mine near Haneti (figure 1). This mine is operated by Dimitri Mantheakis, who hosted two of the authors (JES and BML) there in May 2008. Although both materials have also been found in several other parts of the world (table 1), Tanzania is probably the most important source of chrysoprase after Australia.