corundum, sapphire, Montana.12, USA.12, gems

Crocker reviews the discovery, geology, mining methods, commercial history, and gem characteristics of the Yogo sapphire deposit in Judith Basin County, Montana. Situated near the Little Belt Mountains, the deposit occupies a nearly vertical igneous dike cutting limestone and extending approximately five miles eastward from Yogo Gulch. Its average width is reported as about eight feet, with local widths exceeding twenty feet. Sapphire was recognized during gold prospecting in 1894–95, when blue stones retained in sluice boxes were eventually submitted to Tiffany & Company for evaluation. Subsequent discoveries around animal burrows helped prospectors locate the sapphire-bearing dike after the limited placer deposits were depleted. The New Mine Sapphire Syndicate of London acquired claims in 1897, while a separate operation developed the western portion of the deposit; the syndicate purchased that property in 1914. Fine stones were cut in France and Switzerland, and smaller or inferior material supplied watch jewels, instrument bearings, phonograph needles, and abrasives. Recovery depended heavily on the decomposition of the host rock. Initially, excavated material was exposed on large platforms, soaked, and repeatedly washed until weathering liberated the sapphires; harder rock could require about four years to disintegrate fully. Sluicing, electromagnetic removal of pyrite, and hand sorting completed the process. The western operation instead employed crushing, sizing, gravity concentration, and electrostatic separation before final hand cleaning. Crocker reports an estimated thirteen million carats recovered during thirty-two years of syndicate operations, with approximately fifteen percent by weight classed as gem quality. Financial difficulties associated with taxation and declining sterling, followed by a destructive cloudburst in 1927, contributed to closure. Later attempts to transfer ownership culminated in a reported acquisition in July 1956, with renewed work anticipated in 1957. The article emphasizes the predominantly small size of Yogo sapphires: most weigh less than one carat, and larger crystals are exceptionally uncommon. Colors range from pale steel blue to violet-blue or violetish blue, with occasional green, pink, alexandrite-like, and star material reported. Their attractions include even color distribution and retention of color under artificial illumination. Crocker also describes a fluorescence distinction under a fluorescent lamp, contrasting the moderate violet appearance of Yogo material with the stronger red-orange appearance of comparable Sri Lankan sapphire. Drawing on a U.S. Geological Survey account, he notes etched and rounded crystal surfaces and a thin, dark coating on sapphires in fresh dike rock, interpreted as evidence of reaction with the enclosing magma. Photographs document the dike, workings, and deteriorated mine facilities.