Rock Creek Sapphires

Gems from Montana

A spectrum of color in sapphires from the Rock Creek Mine, Montana, USA. Photo by Aaron Celestian.

Montana sapphire mining has a long and interesting history. Sapphire deposits in Big Sky Country have been known since the late 1800s, with four main areas being of interest: Missouri River, Dry Cottonwood Creek, Yogo Gulch, and Rock Creek. Potentate Mining LLC is currently mining sapphires and gold from Rock Creek, and the company has both loaned and donated some spectacular stones to the Natural History Museum of Los Angeles. I am very excited to have these specimens here.

Sapphire is a gem term for the mineral corundum. Corundum is built from the hexagonal arrangement of aluminum and oxygen, producing classic hexagonally-shaped crystals. Sapphires come in many colors, but not red. Gem-quality red-colored corundum is called ruby. The colors of sapphires come from elemental impurities. For example, blue sapphires have both titanium and iron impurities. Chromium is the source of the red color in rubies.

Donation to NHMLA of rough sapphire crystals from Rock Creek. Photo by Stan Celestian.

Last year, we received a donation of rough stones in a wide array of colors totaling about 75 cts. These stones still retain much of the original crystal habit (growth form), which is unusual for alluvia (river) deposits. Since many sapphires are mined from river deposits, weathering during erosion in the stream rounds out the crystal edges and gives the minerals a frosted appearance. The Rock Creek sapphires likely come from a mudflow that did not travel far from the original sapphire-bearing rock. Because of this lack of substantial transport, the crystal habit of these stones has been preserved.

A 59.5 ct unheated rough sapphire from Rock Creek. As of June 2018, this is the largest crystal that I know of from Montana. A slightly larger one has been found recently. Photo by Aaron Celestian.

One of the sapphires on loan to us is a huge 59.5 ct unheated rough crystal. Color zoning is clearly visible in the hexagonal specimen, with light blue outer edges and a yellow interior. The crystal is rather thin (about 1/4”), so it is unlikely to be cut into a gem. What makes this sapphire so unique is the sheer size of it. It is the biggest sapphire that I’ve seen from Montana.

Sapphires are more than just gemstones; they are used for many applications. Because corundum is one of the hardest minerals, it is much more resistant to scratching then quartz, so many expensive watches use corundum as the face crystal. Sandpaper often has corundum as the abrasive. Corundum is used in the ceramic industry because it tends to be less reactive with other substances at high temperature. Corundum is used in many chemical and industrial applications (sapphire tubing, knives, specialized windows). All those applications use synthetic sapphire. Therefore, nearly all the natural sapphire mined today is for the gem industry, making gemstone wholesale and gem cutters the largest customer for Potentate Mining LLC.

The “Montana Princess” is a 12.62 cts heated blue-green sapphire from Rock Creek. As far as I know, it is the largest blue sapphire gem from Montana. Photo by Aaron Celestian.

So, you might ask if you can see these special stones at the Natural History Museum in Los Angeles. Not currently, but we are putting together a special temporary exhibit for Winter 2018–2019, and it is very likely that these stones will be in the exhibit. More to come on the specifics of the winter dazzle at NHMLA.

Visit us here to learn more about the Gem & Mineral Council at the Natural History Museum of Los Angeles, and how you can be a part of it!

Aaron Celestian is the Mineralogy Curator at the Natural History Museum of Los Angeles. He researches how minerals interact with their environments and with living things, and how those minerals can be used to solve problems like climate change, pollution, and disease.