It is believed that Yogos are in the crown jewels of England and the engagement ring of Princess Diana. Millions of carats were exported, cut and sold in Europe. The vast majority of the previous production was accomplished by the British Syndicate between 19. Fine jewelry containing Yogos was custom manufactured and given to First Ladies Florence Harding and Bess Truman. It is estimated that at least 28 million carats (5.6 tons or 5.5 long tons or 6.2 short tons) of Yogo sapphires contained in approximately 1,500,000 tons of previously unmined dike believed to exist between the old American mine and the British mine. Because Yogo sapphires occur within a vertically dipping resistive igneous dike, mining efforts were historically focused on dike material readily available from surface mining. They have high uniform clarity and are unique because they maintain their brilliance under artificial light. Yogos are typically cornflower blue, which is a result of trace amounts of the elements of iron and titanium. Yogo sapphires are a variety of corundum found only in Yogo Gulch, part of the Little Belt Mountains in Judith Basin County, Montana, United States. Yogos that cut out to two carats or larger are rare and extraordinarily beautiful. However, stones that cut to one carat or more are expected to increase as the depth of the proposed underground mining in initiated in the future. Many of the corundum crystals tend to be flat.
Yogos have high clarity, few if any inclusions, no color zoning and when properly cut they are brilliant. Yogo sapphires range in colors from deep cornflower blue to lilac. Yogo sapphires have been mined on and off since the late 19th century.
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