Because Yogo sapphires occur within a vertically dipping resistive igneous dike, mining efforts have been sporadic and rarely profitable.

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".

The Rock Creek location, near Phillipsburg, is the most productive site in Montana, and its gems inspired the name of the nearby Sapphire Mountains.

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.

East of the Judith River is Pig-Eye Basin, where Jake Hoover, credited as the person who discovered Yogo sapphires, owned a ranch.

[31] 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.

[31][37] 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 dike intrudes Mississippian age (360 to 325 mya) limestone and other sedimentary rocks of the Madison and Big Snowy Groups.

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.

The overlying shale, the Kibbey Formation, was deposited on an unconformity, an ancient Mississippian-age karst erosion surface,[39] and was not intruded by the dike.

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.

[38] Yogo sapphires show crystalline formation under very high temperatures and pressures corresponding to a great depth, over geologically long periods of time.

[18] While millions of carats of sapphires have been mined from the Missouri River deposits, there has been little commercial activity there since the 1990s because of the high cost of recovery and environmental concerns.

The Rock Creek area, also known as Gem Mountain, continues to be the most productive site in Montana, even more so than Yogo Gulch, producing over 190,000,000 carats (38,000,000 g) of sapphires since its inception in 1906.

[50] Millie Ringold, a former slave born in 1845,[52] settled in Fort Benton, Montana after having worked as a nurse and servant for an army general.

[59] A variation is that the teacher lived in Maine, but was a friend of a local miner, who had mailed her a small box with some gold and a few "blue pebbles" in it.

[44] Another story credits a miner named S.S. Hobson for surmising that the blue stones might be sapphires, and his guess was confirmed by a jeweler in Helena.

[44] Ultimately, in 1895, Jake Hoover sent a cigar box containing those he had collected while mining gold to an assay office, which in turn sent them via regular, uninsured mail to Tiffany's in New York City for appraisal by Dr. George Frederick Kunz,[18] the leading American gemologist of the time.

The sale was finally completed for $65,000 cash and some stock considerations because the company's capital was exhausted, similar to previous Yogo ventures.

[71] From 1959 to 1963, the mine itself was left unattended and unsecured, resulting in hobbyists, picnickers, and rockhounds' coming from all over the US and Canada to gather loose rough sapphires.

[74] Siskon bought the mine at a sheriff's sale and in turn leased it to a group headed by Arnold Baron, who had a background in gemcutting and jewelry.

Having done no significant mining or marketing, Sapphire Village, Inc. sold in 1973 to one of its investors, Chikara Kunisaki, a celery farmer from Oxnard, California.

Besides inherent difficulties with financing and the challenges of hard rock mining, the American owners generally did not understand how to effectively market the gems.

Even though mine profits had been poor over the decades, prices of precious gems were very high at the time due to the worldwide oil crises of the 1970s and early 1980s.

Thai traders had even purchased large quantities of naturally colorless Sri Lankan sapphires, known as geuda, and heated them to turn them into a marketable range of blue colors.

Intergem had over $1 million in sales lined up for the 1985 Christmas season, but could only fill a tiny portion because they did not have enough operating capital to manufacture the Yogo jewelry.

[83] Various companies attempted to lease the mine from Roncor, but in the meantime, two local couples, Lanny and Joy Perry and Chuck and Marie Ridgeway, discovered a new site at Yogo Gulch in January 1984 by following a trail to an unused section of the dike that had previously been deemed unsuitable.

Citibank also had obtained a large stock of Yogo sapphires, reputedly worth $3.5 million (approximately $9,915,200 as of 2025), as a result of Intergem's collapse: 200,000 carats (40,000 g) of rough, 22,000 carats (4,400 g) of cut gems, and 2,000 pieces of jewelry, all of which sat in the bank's vaults until 1991 when Sofus Michelsen, director of the Center for Gemstone Evaluation and creator of the Michelsen Gemstone Index, became interested.

[98] An entry of uncut loose Yogo sapphires also won a bronze medal at the 1904 Louisiana Purchase Exposition in St. Louis, Missouri.

In 1952, Gadsden gave cut Yogo sapphires to President Harry Truman, his wife Bess, and their daughter Margaret.