PHYSICAL PROPERTIES
Chemistry: Al2O3
Composition: Aluminum Oxide
Class: Oxides and Hydroxides
Group: Hematite
Crystal System: trigonal; bar 3 2/m
Fracture: conchoidal
Hardness: 9
Specific Gravity: 3.99 - 4.0
Refractive Index: 1.757 - 1.779
Luster: vitreous to adamantine
Streak: white
Color: Most any color some stones show zoning
Cleavage: is absent, although there is parting which occurs in three directions
Transparency: Crystals are transparent to translucent
Associated Minerals: calcite, feldspars, garnets, micas and zoisite
 
ENVIRONMENT:
The Yogo Dike was formed millions of years ago. In central Montana's. Little Belt Mountains. Rocks in the earth's mantle, deep beneath the crust, melted to form molten magmas. Some of those magmas contained materials foreign to the continental crust. One of these magmas rose into the Madison Limestone deposit, where it slowly cooled to form a dike an average of eight feet in width, but over five miles long. As the magma in the dike crystallized, atoms of oxygen combined with atoms of aluminum to form corundum, the mineral form of aluminum oxide. the corundum formed tiny, perfectly shaped transparent crystals rather than the usual blue gray prisms.

CRYSTAL DESCRIPTION:
Yogo sapphires are unique in that they have a very uniform, intense color and thus never require heat treatment. Virtually every crystal contained traces of iron and titanium that gave each crystal a beautiful cornflower blue color. The vast majority of Yogos are this cornflower blue and medium toned purple stones are occasionally found. Yogo Sapphire is said to be the most precious gemstone mined in the United States.

LOCALITIES:
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.

FACTS & HISTORY:
Montana Sapphire is one of the state's gemstones. Indoctrinated in 1969 along with Montana Moss agate. Sapphire's fame began a century earlier in the 1860's. Unpopular at the time because the small multicolored pebbles clogged gold sluices. In a time when, Gold, was the treasure of the day.

Eventualy the US Geological Survey termed the location "America's most important gem location." The British controlled the mines for nearly 30 years, explaining why the beautiful "Cornflower Blue" Yogo's are found in the Royal Crown Jewel Collection in London.

Over the years the Yogo mines have produced an estimated 40 million dollars in the precious blue gems. The original Yogo mines are no longer being worked by commercial companies. The largest cut Yogo is 10.2 carats and is in the Smithsonian Institution in Washington, DC.