Montana has been producing sapphire for more than a century and a half, from four main areas in the southwest of the state. The upper Missouri River gravel bars came first, in 1865, then Dry Cottonwood Creek in 1889, Rock Creek in 1892 and Yogo Gulch in 1895. Today the Missouri River bars and Rock Creek are the ones being actively mined; Yogo Gulch and Dry Cottonwood Creek have been suspended for many years.
Gem Mountain sits in the Rock Creek district, in Sapphire Gulch near its head, off the West Fork of Rock Creek in Granite County. The USGS lists the mine with corundum and gemstone as its primary commodities and the deposit as alluvium of Holocene age, and it records the district itself as producing gold, gemstone and corundum together, with silver and copper behind them. That mix is the giveaway: people were working this ground for gold and finding sapphire in the same gravel.
One line in the USGS file explains the whole trade. The sapphires here are naturally pale, but can be heat treated to reach a deeper blue. Almost every Rock Creek stone you will see set in jewellery has been through a furnace.
The geology is not the riverbed you would expect. Gems and Gemology describes the district's deposits as debris flow, colluvium and secondary alluvial material, which is to say the stones were carried down slope in mudflows before any river got hold of them.
The scale changed recently. In 2015 Potentate Mining consolidated roughly 3,000 acres covering the Rock Creek district near Philipsburg, the first time since the 1890s discovery that one company had assembled a land position that large, taking in the old workings and the hills between them.
Sapphire (every colour)
Other places on the map within a couple of hours.
