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

Yogo Gulch, discovered over a century ago, is one of four major sapphire-producing regions in Montana, United States. Yogo sapphires are celebrated for their uniform, richly saturated blue color, minimal inclusions and zoning, and exceptional luster and brilliance under both natural and artificial light; notably, they require no heat treatment. The rough crystals are typically flat with low cutting retention and usually weigh less than one carat, though specimens up to 19 carats have been reported. Unlike the other Montana sapphire sources, which are secondary deposits, Yogo sapphires are extracted directly from a lamprophyre host rock. At least six dikes are known in the Yogo area, five of which bear sapphire. Between 1895 and 1994, the Yogo deposit yielded an estimated 18.2 million carats of rough, from which over 500,000 carats of cut gemstones are believed to have been produced, and significant reserves remain.