Yogo sapphire

Yogo sapphire
A 0.65-carat (0.130 g) AAA quality cornflower blue Yogo sapphire
General
Category	Oxide mineral
Formula (repeating unit)	Aluminium oxide, Al 2O 3
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (32/m)
Space group	R3c
Identification
Color	Cornflower blue to purple
Crystal habit	Hexagonal, rhombohedral, prismatic or dipyramidal
Twinning	Lamellar
Cleavage	Partings on {0001} and {1011}
Fracture	Uneven to conchoidal
Tenacity	Brittle
Mohs scale hardness	9.0
Luster	Adamantine to vitreous
Specific gravity	3.98–4.10
Optical properties	Uniaxial (–) Abbe number 72.2
Refractive index	nω=1.767–1.772 nε=1.759–1.763, Birefringence 0.008
Pleochroism	Weak
2V angle	58°
References

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 (5.6 t or 5.5 long tons or 6.2 short tons) 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.