Why We Love Montana Sapphires and Yogo Sapphires


montana sapphires and yogo sapphires - matching rings
A great Montana sapphire story: the couple that mined these stones had them cut in Montana and created these matching rings. © CustomMade. Used with permission.

Montana, the Treasure State, produces nearly all of the sapphire in the United States. With rich cornflower blue Yogo sapphires and fancy-colored gems in any color of the rainbow, these stones are a great choice for any jewelry project. A nearby source and an assurance of ethical mining practices make Montana sapphires and Yogo sapphires popular with American consumers.

What are Montana Sapphires and Yogo Sapphires?

A Montana sapphire is, simply, a sapphire from Montana. Yogo sapphires are also Montana sapphires, but originate from a particular deposit known for its excellent cornflower blue color.

Both sapphires and rubies are gem-quality corundum. Rubies are the red variety, while all other colors are sapphires.

With a hardness of 9 on the Mohs scale, only diamond can scratch a sapphire. Furthermore, sapphire doesn’t fracture easily, making it one of the best jewelry stones.

Montana sapphires and yogo sapphires - rough cut and set
The rough and cut sapphires in this image are all natural-color stones from the Missouri River’s El Dorado Bar. Just below the 1.53-ct sapphire ring is the largest stone, weighing in at 10 carats rough.  In all, the rough in this photo is 64 carats.  © Earth’s Treasury. Used with permission.

In terms of gem production in the United States, sapphire is commercially mined only in Montana. Fee digging sites in North Carolina also produce some sapphire, but only a small amount of sapphire occurs in North Carolina.

History and Geology of Montana Sapphires

Four areas of Montana produce gem-quality sapphire. In 1865, gold rush prospectors discovered sapphire in the gravels of the Missouri River. At the time, these stones were waste that got in the way of extracting gold because no facilities for faceting and polishing sapphire existed anywhere nearby. As a result, these pebbles were nearly worthless. Nowadays, these deposits produce the largest of Montana sapphires, with 20 carat specimens not unheard of.

montana sapphires and yogo sapphires - El Dorado Bar
A view of snow-covered tail minings from the El Dorado Bar, one of the Missouri River sapphire deposits. “Sapphire Mine Tailings on El Dorado Bar, Hauser Lake, Montana, 2015-12-27” by Tim Evanson. Licensed under CC By-SA 2.0.

Prospectors discovered sapphire in Rock Creek and in Dry Cottonwood Creek soon after. These three areas produce pale-colored, but often high clarity, rough. Because of the light colors, mining these areas wasn’t very profitable before heat treatment techniques were perfected.

To complicate matters, the original geological source area for these gems is currently unknown. One theory suggests that ancient sapphire-bearing mountains weathered away, with streams carrying these gems downhill. They were then buried in ancient streambeds. These ancient streambeds are now weathering away, and water carries these sapphires into modern riverbeds. Another theory suggests that long-distance glacial transport played a role. Either way, geologists have yet to discover the original source of these gems.

Yogo Sapphires

Stones from the famous Yogo Gulch area, on the other hand, occur in an igneous host rock. Miners can go directly to the source area to find these stones, rather than sifting through streambed sediments. These Yogo sapphires have been mined on and off for the last century.

montana sapphires and yogo sapphires - diamond and yogo sapphire ring
Because Yogo sapphires tend to be small, jewelry makers rarely use them as center stones in rings. However, their size makes them perfect side stones. © CustomMade. Used with permission.

In 1984, the owner of Yogo Gulch mining operations, Dennis Brown, started a rumor that Princess Diana’s engagement ring was, in fact, a Yogo sapphire unearthed during British control of the mine. Though this is unconfirmed and unlikely, this marketing stunt certainly didn’t hurt the Montana sapphire.

Today, Yogo Gulch is an inactive mine. As a result, these gems can be quite expensive. However, the source rock isn’t depleted.  It’s only a matter of time before more of these gems are unearthed.

What Colors are Montana Sapphires and Yogo Sapphires?

Because of their pale color, most Montana sapphires weren’t worth very much prior to the development of heat treatment. Still, some stones have attractive colors prior to treatment. In the Rock Creek deposit, approximately 15% of rough is a marketable color prior to treatment. This includes shades of blue, yellow, and pink. Still, Montana sapphires, like many sapphires on the market, often have a “steely” grey component to their color.

montana sapphires and yogo sapphires - rock creek
Greenish blue and steely, this Rock Creek sapphire is a typical color for this deposit. Photo by Astynax. Public domain.

Some Montana sapphires from the Missouri River deposit undergo color change. Color change in these sapphires results from the trace element vanadium. Often, they appear blue in daylight and violet to purple in incandescent light. Pale colors shift from sky blue to lavender.

Yogo sapphires have a beautiful deep cornflower blue color without any treatment. However, these stones tend to be small, with few above 0.5-ct. Most notably, these sapphires generally lack color zoning, a common phenomenon in sapphire. As a result, these are very fine and untreated specimens, and collectors seek out large, rare sizes.

montana sapphires and yogo sapphires - yogo sapphire
At 0.65-ct, this Yogo sapphire is large for its locality, and displays the typical deep cornflower blue. Image by Montanabw. Image taken at Barnes Jewelry, Helena, MT. Licensed under CC By-SA 3.0.

Only about 2% of Yogo sapphires exhibit a deep purple color. Even more rarely, Montana produces a few rubies. Although sapphire and ruby are the same mineral – corundum – finding both in the same geological deposit is extremely rare.

Montana sapphires and yogo sapphires - montana ruby
This magenta-pink ruby is a true rarity. At 0.97-ct, it is one of the largest rubies unearthed in the El Dorado Bar. © Earth’s Treasury. Used with permission.

Heat Treatment

Most Montana sapphires undergo heat treatment to deepen pale color or to give color to colorless stones. When heated under oxidizing conditions, these gems often become bright yellow. Under reducing conditions, a deep blue hue arises.

montana sapphires and yogo sapphires - treated and untreated
Can you guess which of these stones have undergone heat treatment? The pink and yellow stones have natural colors, and the green and blue stones have undergone heat treatment to enhance clarity and intensify color. © Earth’s Treasury. Used with permission.

Due to high iron content, achieving the proper reducing conditions is tricky. When the atmosphere is too reducing, mineral inclusions may form in these sapphires. This iron content also inhibits lattice diffusion, so this treatment is uncommon for Montana sapphires.

Star Sapphire

A few rare specimens of Montana sapphire have fine silk inclusions that impart asterism. However, star sapphire cabochons are uncommon, in part because sapphire is so desirable in faceted form. As a result, some sapphires which could display asterism are instead faceted.

About the author
Addison Rice
A geologist, environmental engineer and Caltech graduate, Addison's interest in the mesmerizing and beautiful results of earth's geological processes began in her elementary school's environmental club. When she isn't writing about gems and minerals, Addison spends winters studying ancient climates in Iceland and summers hiking the Colorado Rockies.
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