Yogo sapphires are mined in Montana at Yogo Gulch (46°50′45″N 110°18′38″W), which is in Judith Basin County, Montana, 12 miles (19 km) southwest of Utica, 45 miles (72 km) 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 (8,625 feet (2,629 m), 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 3 miles (5 km) north of Yogo Creek's confluence with the Middle Fork of the Judith River; from there it runs east-northeast and ends about 0.5 miles (800 m) from the Judith River. Yogo Creek starts just south of Yogo Peak, which is about 15 miles (24 km) 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.
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 traditional hard rock mining. At a minimum, independent geologist report that there is a minimum of 28,000,000 carats (5,600 kg) are estimated to be unmined. The Yogo dike is "the only known igneous rock in the world from which sapphires can be 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, clasticsedimentary 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 aluminum content provided by assimilation of clay rich shales of the ProterozoicBelt Supergroup sediments which are known to be present at depth in the region.
The Yogo dike is a narrow sub-vertical sheet-like igneous body. It varies from 2 to 26 feet (0.61 to 7.92 m) thick and extends for 5 miles (8.0 km), striking at an azimuth of 255°. The dike is broken into three offset end echelon segments and dates to 48.6 mya using Argon 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 7,000 feet (2,100 m) 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 karsterosion 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. Yogos 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 "carats per ton" for the entire dike could be 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 carats per ton than the English Mine.