In the 1880s, small crystals of sapphire
, partially suitable for jewelry purposes, were found in gold-bearing alluvial placer deposits of the Missouri River basin in Montana. Since that time, the sapphire has been extracted with gold as a by-product. Some placer deposits were mined solely for gem production. The primary sources of sapphire in placer deposits are assumed to be corundum-bearing lamprophyres, about 150 km to the east-northeast, or biotite trachytes from the Helena region.
Ref: E.Ya. Kievlenko (2003) Geology of gems, p. 69