Yogo Sapphires at Moccasin Mountain
300 million years ago, an ancient sea covered much of what is now Montana. The seabed, under geologic pressure, formed the Madison limestone layer. Then, 250 million years later, uplift created the Little Belt Mountains, which lie about 35 miles west and south of Lewistown. Magma from the earth’s mantle rose into a fracture in the limestone and formed a five-mile long, ten-foot wide dike of lamproite—the Yogo Sapphire’s host rock. Geologic surveys show this dike is 7,000 feet deep, making it the world’s largest known sapphire deposit. Mining efforts to date have never extended deeper than 400 feet, yet Yogo Sapphires are the most precious gemstones ever to be mined in the U.S.
Yogos sapphires range in color from violet to cornflower blue, though it is the distinct beautiful blue stones and their exceptional clarity that give Yogos their worldwide prestige. This coloring comes from traces of iron and titanium. Queen Victoria, the Duchess of York, Lady Diana of England, and Kaiser Wilhelm of Germany have all owned them. They are in collections at the Smithsonian Institute in Washington, D.C, at the Museum of Natural History in New York, and are part of the British Royal Crown jewel collection in London. In Montana, Yogo sapphires have special allure, as Yogo Gulch, just up the road from Utica, an hour’s drive west of Lewistown, is the only place in the world where Yogo Sapphires are to be found.
Yogo sapphires are naturally almost flawless. Gemstones deemed “Montana Sapphires” cannot hold a candle to a true Yogo. “Montana Sapphires” must undergo heat treatment to be rid of impurities and even then cannot match the color and beauty of the Yogo Sapphire. Although “Montana Sapphire” stones may be larger, they never obtain the pure true blue of the Yogos. As to the controversial matter of heat treatment, no sapphires in the world can challenge the quality of the sapphires that come from the Yogo dike. The standard of excellence among sapphires is right where it belongs—in Montana at the Yogo dike.
Although well-cut Yogos larger than one carat are rare, exceptional gems have been known to sell for up to $100,000. The Yogo dike has produced more than $25,000,000 of the world’s most beautiful sapphires; but Yogos are embedded in hard rock, which makes mining them difficult and expensive. At present, the mine sits abandoned and deserted. Nearby, the Sapphire Village subdivision allows property owners lifetime digging rights on the old mine. For them, rock hounding is a hobby, and residents spend summers collecting buckets of old mine dirt, leaked from flumes that sat atop the hill above Yogo Gulch nearly 100 years ago.