Gem-Quality East African Prase Opal. Mount Iyobo, Tanzania

Gem-Quality East African Prase Opal. Mount Iyobo, Tanzania © 2002-2010 Bonhams 1793 Ltd

Weighing approximately 28.35 carats and measuring 22.5 x 19.0 x 11.0mm - Sold for $3,050

Both chrysoprase and prase opal have been mined on Iyobo Mountain in central Tanzania for more than 20 years, with the first discoveries having been described by Dr. Edward Gübelin in 1975. Prase opal is a nickel-bearing variety of common opal, where nickel is the chromophore (the part of a molecule responsible for its color) that lends the neon bluish-green hue a luminous quality. Chrysoprase and prase opal can be similar in appearance but can easily be distinguished by their refractive index and specific gravity. In addition, prase opal is more translucent and gemmy. A recent discovery in 2009 revealed some of the most intense and beautiful bluish green material ever to reach the gem market. The majority of the production originates in veins within metamorphosed rocks and is commonly found in chrysoprase form: prase opal is the rarer find from this deposit. Extraordinarily brilliant apple green color and a high degree of gemminess distinguish this fine translucent oval cabochon jewel making it suitable for the serious collector.
Footnote: This recent discovery of prase opal was written up in Gems and Gemology, Winter 2009.

Bonhams & Butterfields. The Nature of Opals, 10 Nov 2010. 220 San Bruno Avenue, San Francisco www.bonhams.com