- Chemical Formula: Al2O3
- Mohs Hardness: 9
- Locality: Yogo Gulch, and Missouri River or Rock Creek, Montana
These sapphires belong to the gemstone family corundum. They are chemically aluminum and oxygen, but their color is derived from inclusions of other elements—iron, gallium, and magnesium. Red corundum stones contain chromium, and are called rubies!
The Yogo Gulch sapphire deposit was found in 1895, by prospectors looking for gold. The sapphires they found turned out to be much more valuable, and mining began in Yogo Gulch not long after. Some 28 million carats are believed to still be buried in those hills waiting to be discovered.
All other sapphires from Montana, like the Missouri River and Rock Creek stones, came from an unknown primary source vein. They collected in the areas where they were found through millions of years of erosion. As the river carved a canyon through erosion, the sapphires were exposed.