The Yogo Gulch sapphire deposit is located on the northeastern side of the Little Belt Mountains in Judith Basin County, about 74 miles east-northeast of Helena.
Commercial mining for sapphires at Yogo began in 1896. Charles T. Gadsden, an English mining engineer, originally oversaw the operation at Yogo. By the time the mine was closed in 1929, Yogo had produced 2.5 million carats of gemquality sapphires, reportedly worth as much as US$25 million. For further information, the reader is referred to Voynick (1987); Mychaluk (1995); and Hughes (1997).
The original Yogo mine, which has been owned by Roncor Inc. since 1972, was most recently mined during 2000 and 2001 by a publicly traded Canadian company named Pacific Sapphire. They have since ceased operations and divested themselves of their sapphire interests. At the present time, Roncor has no plans to resume mining. For almost 100 years, miners have worked material relatively near the surface. Today, however, most of this area has been depleted, so any future large-scale operations must go deep underground, which is more difficult and much more expensive.
In 1984 a group of local prospectors formed the Vortex Mining Company and discovered several new sapphire-bearing dykes on the west end of the Yogo deposit. They began mining in 1987 and are the only company mining at Yogo today; now under the name Yogo Creek Mining. They are mining underground, via a 260 feet deep shaft. They told the author recently that production is very good, with demand much higher than they are able to meet, especially in 1-carat-plus round brilliants. Although the color of Yogo sapphires is often among the very best in the world, the crystals typically are small less than 10% weigh more than one carat and have a characteristic flat, tabular shape that makes it difficult to retain a high percentage of weight during cutting.
Yogo sapphires are unique in that they have a very uniform, intense color and thus never require heat treatment. The vast majority of Yogos are blue (historically referred to as “cornflower”), although medium-toned purple stones are occasionally found.
Rock Creek, or Gem Mountain as it is popularly known, is located near the town of Philipsburg in Granite County, about 80 miles west-southwest of Helena. The deposit runs principally along two tributaries of the west fork of Rock Creek: Anaconda Gulch and Sapphire Gulch.
Rock Creek is unique in that it is the only placer deposit in Montana that was mined before World War II solely for sapphires (the other areas were mined for their considerable gold content). Mining commenced at Rock Creek around 1906 and was fairly steady until 1943, when synthetic corundum replaced natural sapphire for many commercial applications. Mining records indicate that 190 million carats of sapphire were shipped from Rock Creek between 1906 and 1923. Rock Creek has been mined more extensively and produced more faceted gems during its 110 year history than any of the other sapphire deposits in Montana (including Yogo).
In this author’s opinion, Rock Creek is the most important of the Montana secondary deposits. The mountain is covered with sapphire that is easy to mine (there is a reason that the entire mountain range is named the “Sapphire Mountains”); it heat treats extremely well, producing a full range of colors; and cutters get high weight retention from the crystals (figure 4). Some blue and fancy-color sapphires found there are attractive without heat treatment.
AGC owned the majority of the sapphire bearing ground at Rock Creek from 1994-2001. AGC divested themselves of their sapphire interests in 2001, selling the mine to one concern and all of the other sapphire-related assets, including large inventories of rough and cut sapphires, to Fine Gems International (a wholesale company specializing in fine gemstones from around the world).
Currently, the largest property owner (of nearly 500 acres of mining claims) is operating a successful tourist operation, selling buckets of sapphire-bearing gravels. He plans to begin small-scale mining in 2003, with three people. In between and adjacent to this property are several other individuals or small companies that own mineral rights and sporadically mine in groups of two or three. The severe winters restrict mining to the period between mid-May and mid-October. Another company offers tourists and sapphire enthusiasts the opportunity to enjoy the spectacular area by taking a sapphire mining pack trip on horseback.
Robert Kane is the former Director of the Gübelin Gem Lab in Lucerne, Switzerland and the former Manager of Gem Identification at GIA’s West Coast Gem Trade Laboratory. He is currently President & CEO of Fine Gems Inter-national, and can be contacted at firstname.lastname@example.org. © 2002 Robert E. Kane