Why We Love Montana Sapphires and Yogo Sapphires

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.

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.

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.

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.

Some Montana sapphires from the Missouri River deposit can show 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 carats. 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.

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.

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.

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, the "star stone" effect. 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.