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...