Yogo sapphires are blue sapphires, a colored variety of corundum, found in the US state of Montana, primarily in Yogo Gulch (part of the Little Belt Mountains) in Judith Basin County. Yogo sapphires 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 Yogo sapphires are still in the ground. Jewelry containing Yogo sapphires was given to First Ladies Florence Harding and Bess Truman; in addition, many gems were sold in Europe, though promoters' claims that Yogo sapphires 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.
"Yogo sapphire" is the preferred term 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 Yogo sapphires 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 Yogo sapphires 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.
Etymology Because Yogo Gulch lies in a region historically inhabited by the Piegan Blackfeet people, promoters of Yogo sapphires claim that yogo may mean "romance" or "blue sky" in the Blackfoot language, although there is little evidence to support this claim. Other meanings for yogo have been suggested, including "Going over the hill". The meaning of the word "Yogo" had been lost by 1878, when placer gold was found in Yogo Creek. Thus, its true meaning is uncertain.
Mineralogy and geology
Sapphires are a color variety of corundum, a crystalline form of aluminium oxide (). Corundum is one of the hardest minerals, rating 9 on the Mohs scale. Corundum gems of most colors are called sapphires, except for red ones, which are called rubies. The term "Yogo sapphire" refers only to sapphires from the Yogo Gulch. The cornflower blue color of the Yogo results from trace amounts of iron and titanium. Yogo sapphires are unique in that they are free of cavities and inclusions, have high uniform clarity, lack color zoning, and do not need heat treating because their cornflower blue coloring is uniform and deep. Unlike Asian sapphires, they maintain their brilliance in artificial light. Yogo sapphires present an advantage to gemcutters: since they are found as primary constituent minerals within an igneous bedrock rather than in sedimentary alluvial deposits where most other sapphires are located, they retain a perfect or near perfect crystalline shape, making cutting much easier, as does their lack of inclusions, color zoning, or cloudiness. Yogo sapphires also exhibit a triangular pattern on the basal plane of the flattened crystals, with thin rhombohedral crystal faces, a feature absent in sapphires from other parts of Montana.
Yogo sapphires tend to be beautiful, small, and very expensive. The United States Geological Survey and many gem experts have stated that Yogo sapphires are "among the world's finest sapphires." The roughs tend to be small and flat, so cut Yogo gems heavier than are rare. Only about 10 percent of cut pieces are over . The largest recorded Yogo rough, found in 1910, weighed and was cut into an gem. The largest cut Yogo is . Because of the rarity of large rough Yogo sapphires, Yogo gem prices begin rising sharply when they are over , and skyrocket when they are over .
Montana sapphires in general come in a variety of colors, but Yogo sapphires are almost always blue. About two percent of Yogo sapphires are purple, due to trace amounts of chromium. A very small number of rubies have been found at Yogo Gulch.
Yogo sapphires were first discovered in alluvial streambed sediments during gold mining operations in Yogo Gulch downstream from the Yogo dike, but were later traced to their source within igneous bedrock. Worldwide, other than the Yogo Gulch deposit and one small site in the Kashmir region, most other corundum is mined from the sand and gravel created by the weathering of metamorphic rock. Alluvial sapphires are found in the Far East, Australia, and in three other Montana locations—the upper Missouri River, Rock Creek, and Dry Cottonwood Creek. The location of most Yogo sapphires within igneous rock rather than from alluvial placer deposits requires difficult hard rock mining. Coupled with American labor costs, this makes their extraction fairly expensive. At least are estimated to still be in the ground. The Yogo dike is "the only known igneous rock from which sapphire is mined".
The sapphire bearing Yogo dike is a dark gray to green intrusive rock known as a lamprophyre. The lamprophyre is an unusual igneous rock that contains a low content of silica. The rock has a porphyritic texture with large crystals of orthopyroxene and phlogopite set in a fine grained matrix. The phlogopite crystals have been used to determine the age of the dike and its crystallization temperature (900 °C (1,650 °F)). The dike also contains fragments of other rock types. These xenoliths include pieces of limestone, clastic sedimentary rocks, and gneiss. In some locations, due to the abundance of xenoliths, the dike has the appearance of a limestone breccia in an igneous matrix. One gneiss fragment found as a xenolith contains corundum. The Yogo sapphires themselves are rimmed with a reaction layer of spinel and are etched, indicating that the sapphires were not in chemical equilibrium with their host, the lamprophyre magma. This suggests the sapphire crystals may have originated in an earlier rock, such as a corundum-bearing gneiss, later assimilated by the lamprophyre magma at depth. Earlier investigators had assumed that the sapphire had crystallized from the magma with the necessary high aluminium content provided by assimilation of clay rich shales of the Proterozoic Belt Supergroup sediments which are known to be present at depth in the region.
The Yogo dike is a narrow subvertical sheet-like igneous body. It varies from thick and extends for , striking at an azimuth of 255°. The dike is broken into three offset en echelon segments, and dates to 48.6 mya using Ar dating on phlogopite. The dike intrudes Mississippian age (360 to 325 mya) limestone and other sedimentary rocks of the Madison and Big Snowy Groups.
There has been considerable debate over the years as to the depth of the Yogo dike and how many ounces of rough sapphires per ton it contains. In the late 1970s and early 1980s, Delmer L. Brown, a geological engineer and gemologist, conducted the most thorough scientific exploration up to that time, concluding that the dike was at least deep and that the concentration of rough sapphires was not constant throughout the deposit. Brown found that the dike had intruded into a pre-existing fault that had been a conduit for groundwater circulation. The overlying shale, the Kibbey Formation, was deposited on an unconformity, an ancient Mississippian-age karst erosion surface, and was not intruded by the dike. This groundwater action produced collapsed zones which were intruded by the dike to form breccia zones. Recent erosion in the area removed the overlying shales and again exposed the limestone to groundwater action which produced collapse breccias which include fragments of the dike rock. He determined that the erosion of the dike in the current erosion cycle was minimal.
Brown also showed that the unique characteristics of the Yogo sapphires are related to their geological history. Most sapphires are formed under low pressure and temperature over geologically short periods of time, and this is why most non-Yogo sapphires have imperfections and inconsistent coloring. Yogo sapphires show crystalline formation under very high temperatures and pressures corresponding to a great depth, over geologically long periods of time. Brown also showed that distribution of gem rough through the dike was not consistent, so using an average "ounces per ton" was misleading. For example, the section which, despite several ownership and name changes over the years, is generally known as the "American Mine," was developed in an area dominated by post-dike breccia with significantly lower ounces per ton than the English Mine.
Montana sapphires