gems, corundum, USA.12, Montana.12, Yogo Gulch

Sapphire-bearing dikes are located in the Little Belt Mountains, Montana. This report gives an accurate and comprehensive review of the sapphire deposit and its history. Yogo sapphires constitute a world-class deposit hosted by an ultramafic ouachitite (lamprophyre) dike system of Eocene age (about 50 million years old) in central Montana. The sapphires are mostly cornflower-blue, have extremely few inclusions, and are not zoned. With high brilliance and luster in both natural and artificial light, they do not require heat treatment to enhance their color. It is estimated that since discovery in 1895 production is 18.2 million carats of rough gems with 500,000 carats of faceted sapphires. Most of the gems are less than 1 carat, platy, and relatively thin, although specimens up to 19 carats have been reported. The largest sapphire with a suitable thickness for faceting as a single gem was 12.25 carats.