Yogo Gulch, discovered more than 100 years ago, is one of four major sapphire-producing areas in Montana, United States. Yogo sapphires are known for their uniform, well-saturated blue color; relative absence of inclusions and zonation; and high luster and brilliance in both artificial and natural light. They do not require heat treatment. Rough crystals, usually flat with low cutting retention, generally weigh less than one carat (but have been reported up to 19 ct). Unlike the other Montana deposits, which are secondary, Yogo sapphires are mined directly from a lamprophyre host rock. There are at least six known dikes, five sappire-bearing, at Yogo. From 1895 to 1994, the Yogo deposit produced an estimated 18.2 million carats of rough that are believed to have yielded more than 500,000 carats of cut stones. Considerable reserves remain.

Appears on pages 28-41