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.
Discovering the Beauty of Montana Sapphires and Yogo Sapphires
A Montana sapphire is simply a sapphire from Montana, while Yogo sapphires are a specific type of Montana sapphire known for their stunning cornflower blue color. Both sapphires and rubies are considered gem-quality corundum. Rubies are the red variety, and all other colors fall under the sapphire category.
With a Mohs scale hardness rating of 9, sapphires are significantly durable and resistant to scratching. Their exceptional toughness also makes them an ideal choice for jewelry.
The rough and cut sapphires in this image are all-natural stones from the Missouri River’s El Dorado Bar. © Earth’s Treasury. Used with permission.
Montana is the primary commercial mining location for sapphires in the United States. While there are some fee digging sites in North Carolina that produce sapphires, the quantity is relatively small compared to Montana.
A Glimpse into the History and Geology of Montana Sapphires
Four regions in Montana are known for producing gem-quality sapphires. The discovery of sapphires in the gravels of the Missouri River by gold rush prospectors in 1865 initially deemed them worthless because there were no facilities nearby for faceting and polishing. However, these deposits now yield some of the largest Montana sapphires, with specimens weighing up to 20 carats.
A view of snow-covered tail minings from the El Dorado Bar, one of the Missouri River sapphire deposits. Photo by Tim Evanson. Licensed under CC By-SA 2.0.
Sapphire deposits in Rock Creek and Dry Cottonwood Creek were discovered shortly after the Missouri River findings. These areas yield rough sapphires with pale colors but high clarity. Initially, mining these areas was not profitable due to the light colors, but with the advancement of heat treatment techniques, their value has increased.
The exact geological origin of these sapphires remains a mystery. One theory suggests that ancient sapphire-bearing mountains weathered away, with streams transporting the gems downhill and burying them in ancient streambeds. Another theory suggests that long-distance glacial transport played a role. Geologists have yet to uncover the original source.
The Enigma of Yogo Sapphires
Unlike other Montana sapphires, Yogo sapphires are found in igneous host rock. Miners can directly access the source area, eliminating the need for sifting through streambed sediments. Yogo sapphires have been mined intermittently for over a century.
Because Yogo sapphires are usually small, jewelry makers often use them as side stones rather than center stones. © CustomMade. Used with permission.
In 1984, Dennis Brown, the owner of Yogo Gulch mining operations, sparked a marketing buzz by suggesting that Princess Diana’s engagement ring featured a Yogo sapphire from his mine during British control. Although unconfirmed, this rumor further enhanced the reputation of Montana sapphires.
Today, Yogo Gulch is no longer an active mine, making Yogo sapphires quite valuable. However, there is still untapped potential in the source rock, and more of these dazzling gems are yet to be unearthed.
Exploring the Variety of Colors in Montana Sapphires and Yogo Sapphires
Greenish blue and steely, this Rock Creek sapphire is a typical color for this deposit. Photo by Astynax. Public domain.
Prior to heat treatment techniques, many Montana sapphires had a pale appearance and were not highly valued. However, some stones exhibited attractive colors even before treatment.
Approximately 15% of rough sapphires from the Rock Creek deposit possessed marketable colors, including shades of blue, yellow, and pink. However, it is worth noting that many Montana sapphires, like other sapphires on the market, have a slight grey component in their color, often referred to as “steely” grey.
Certain Montana sapphires from the Missouri River deposit exhibit color change, which is caused by trace elements of vanadium. These sapphires appear blue in daylight and violet to purple in incandescent light, with lighter shades shifting from sky blue to lavender.
Yogo sapphires, on the other hand, showcase a stunning deep cornflower blue color without any treatment. However, they are typically small, with few exceeding 0.5 carats. What makes them truly exceptional is their lack of color zoning, a characteristic commonly observed in other sapphires. Collectors seek out larger and untreated Yogo sapphires.
This magenta-pink ruby is a true rarity. It is one of the largest rubies discovered in the El Dorado Bar. © Earth’s Treasury. Used with permission.
About 2% of Yogo sapphires feature a deep purple color, and occasionally, Montana produces a few rubies. Although sapphires and rubies are the same mineral, finding them together in the same geological deposit is extremely rare.
Unveiling the Art of Heat Treatment
To enhance the color of Montana sapphires, most undergo heat treatment. This process deepens pale colors and imparts color to colorless stones. Under oxidizing conditions, sapphires often turn bright yellow, while reducing conditions produce a rich blue hue.
Can you guess which of these stones have undergone heat treatment? The pink and yellow stones have natural colors, while the green and blue stones have undergone treatment to enhance clarity and intensify color. © Earth’s Treasury. Used with permission.
Due to their high iron content, achieving the optimal reducing conditions for Montana sapphires is challenging. If the atmosphere is too reducing, mineral inclusions may form, affecting the gem’s clarity. Additionally, the iron content inhibits lattice diffusion, making this treatment uncommon for Montana sapphires.
The Enchanting World of Star Sapphire
Although star sapphires with fine silk inclusions that create an asterism effect exist in Montana, they are relatively rare. The demand for sapphires in faceted form often steers away from cabochons with star effects. Consequently, some sapphires that could display asterism are faceted instead.
These breathtaking Montana sapphires and Yogo sapphires continue to captivate our hearts with their unique beauty and fascinating origins. Each gem tells a story as vibrant as the enchanting colors they possess.
