In 1865, gold prospectors near Helena kept finding strange blue pebbles clogging their sluice boxes while panning the Missouri River. They didn't know what they were and mostly discarded them, since they were after gold, not colored rocks. It took years before the stones were identified as sapphires. That site, now called Missouri River/Eldorado Bar, is known for producing sapphires with a deep, saturated blue.

The bigger discovery came in 1895 at Yogo Gulch, where sapphires impressed a gemologist enough to spark commercial mining; Yogo stones are famous for cornflower blue color that needs little to no heat treatment.

A third deposit, Rock Creek, turned out to be the most productive and yields a huge color range - blues, greens, teals, pinks, yellows - which is why Montana sapphire today is linked to non-traditional gem colors.