|Yogo sapphires are a variety of corundum found only in Yogo Gulch, part of the Little Belt Mountains 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. Because Yogo sapphires occur within a vertically dipping resistive igneous dike, mining efforts have been sporadic and rarely profitable.|
|Gold was discovered at Yogo Creek in 1866, and though "blue pebbles" were noticed alongside gold in the stream alluvium by 1878, it was not until 1894 that the "blue pebbles" were recognized as sapphires.|
Sapphire mining began in 1895 after a local rancher sent a cigar box of gems he had collected to an assay office, which in turn sent them to Tiffany's in New York, where an appraiser pronounced them "the finest precious gemstones ever found in the United States".
|The Yogo dike is a narrow subvertical sheet-like igneous body. It varies from 2 to 26 feet (0.61 to 7.9 m) thick and extends for 5 miles (8.0 km), striking at an azimuth of 255°. |
Montana sapphires were heavily mined during World War II for industrial abrasive and cutting purposes. In the early 1980s a scientific exploration concluded that the dike was at least 7,000 feet (2,100 m) deep and that the concentration of rough sapphires was not constant throughout the deposit.