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Yogo sapphires are a variety of corundum found only in Yogo Gulch, part of the Little Belt Mountains in Judith Basin County, Montana, United States, on land once inhabited by the Piegan Blackfeet people. Yogos are typically cornflower blue, a result of trace amounts of iron and titanium. They have high uniform clarity and maintain their brilliance under artificial light. Because Yogo sapphires occur within a vertically dipping resistive igneous dike, mining efforts have been sporadic and rarely profitable. It is estimated that at least 28 million carats () of Yogos are still in the ground. Jewelry containing Yogos was given to First Ladies Florence Harding and Bess Truman; in addition, many gems were sold in Europe, though promoters' claims that Yogos are in the crown jewels of England or the engagement ring of Princess Diana are dubious. Today, several Yogo sapphires are part of the Smithsonian Institution's gem collection. Yogo sapphires were not initially recognized or valued. Gold was discovered at Yogo Creek in 1866, and though "blue pebbles" were noticed alongside gold in the stream alluvium by 1878, it was not until 1894 that the "blue pebbles" were recognized as sapphires. Sapphire mining began in 1895 after a local rancher named Jake Hoover sent a cigar box of gems he had collected to an assay office, which in turn sent them to Tiffany's in New York, where an appraiser pronounced them "the finest precious gemstones ever found in the United States". Hoover then purchased the original mother lode from a sheepherder, later selling it to other investors. This became the highly profitable "English Mine", which flourished from 1899 until the 1920s. A second operation, the "American Mine", was owned by a series of investors in the western section of the Yogo dike, but was less profitable and bought out by the syndicate that owned the English Mine. In 1984, a third set of claims, known as the Vortex mine, opened. The term "Yogo sapphire" is the preferred wording for gems found in the Yogo Gulch, whereas "Montana sapphire" generally refers to gems found in other Montana locations. More gem-quality sapphires are produced in Montana than anywhere else in North America. Sapphires were first discovered in Montana in 1865, in alluvium along the Missouri River. Finds in other locations in the western half of the state occurred in 1889, 1892, and 1894. The Rock Creek location, near Phillipsburg, is the most productive site in Montana, and its gems inspired the name of the nearby Sapphire Mountains. In 1969, the sapphire was co-designated along with the agate as Montana's state gemstones. In the early 1980s, Intergem Limited, which controlled most of the Yogo sapphire mining at the time, rocked the gem world by marketing Yogos as the world's only guaranteed "untreated" sapphire, exposing a practice of the time wherein 95 percent of all the world's sapphires were heat-treated to enhance their natural color. Although Intergem went out of business, the gems it mined appeared on the market through the 1990s because the company had paid its salesmen in sapphires during its financial demise. Citibank had obtained a large stock of Yogos as a result of Intergem's collapse, and after keeping them in a vault for nearly a decade, sold its collection in 1994 to a Montana jeweler. Mining activity today is largely confined to hobby miners in the area; the major mines are currently inactive. == Location == Yogo sapphires are mined in Montana at Yogo Gulch (), which is in Judith Basin County, Montana, southwest of Utica, west-southwest of Lewistown, and east of Great Falls. The site was in Fergus County when Yogo sapphires were discovered, but in 1920, because of the re-designation of county boundaries, Judith Basin County was carved out from parts of western Fergus County and eastern Cascade County. Yogo Gulch and the corresponding natural features of Yogo Peak (), Yogo Creek, and the Yogo dike, where the gems are mined, are all in the Little Belt Mountains within Judith Basin County. The Gulch is located along the lower reaches of Yogo Creek and west of the Judith River. The west end of the Yogo dike outcrops just southwest of Yogo Creek, about north of Yogo Creek's confluence with the Middle Fork of the Judith River; from there it runs east-northeast and ends about from the Judith River. Yogo Creek starts just south of Yogo Peak, which is about west of the Judith River. From there the creek flows southeast into the Middle Fork of the Judith River. The Judith River then flows northeast from the Little Belts toward Utica. East of the Judith River is Pig-Eye Basin, where Jake Hoover, credited as the person who discovered Yogo sapphires, owned a ranch. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Yogo sapphire」の詳細全文を読む スポンサード リンク
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