|Yogo sapphires are a variety of corundum found only in Yogo Gulch in Judith Basin County, Montana.|
Yogos are typically cornflower blue, a result of trace amounts of iron and titanium. They have high uniform clarity and maintain their brilliance under artificial light. Yogo sapphires occur within a vertically dipping resistive igneous dike.
Mining efforts have been sporadic and rarely profitable.
|Mining of Yogo sapphires remains exceptionally difficult. Even so, Yogo sapphire mining turned out to be more valuable than several gold strikes. The Yogo area also produced small amounts of silver, copper, and iron.|
Montana sapphires were heavily mined during World War II for industrial abrasive and cutting purposes.
There is hobby mining on small parcels, but commercial mining operations of Yogo Sapphire have ended.