Yogo Gulch, Yogo District, Little Belt Mountains, Judith Basin County, Montana, USA
The famous Yogo Gulch sapphire deposit is 72km southwest of Lewistown, Judith Basin County, central Montana. The deposit was developed intensively from 1897 to 1929 and continues to produce unique blue sapphires.The sapphire mineralization is in a large lamprophyric dike more than 6.5km long, and between 1.5m and 7m thick. The dike dips at a steep, almost vertical, angle.
To a depth of 100m and more from the surface the dike is irregularly and locally intensively kaolinised, permitting easy extraction of sapphire. In developed areas cuttable sapphires average around 10 carats per ton of rock. Sapphires are mainly small, up to one carat, but the largest raw crystal weighed 19 carats.
The colour of Yogo Gulch sapphires is dominantly uniform blue, from pale to saturared cornflower colour, sometimes with a distinct violet tint.
Ref: E.Ya. Kievlenko (2003) Geology of gems, p. 47