Gem News International Gems & Gemology, Spring 2017, Vol. 53, No. 1

Magnificent Sapphires from Montana’s Rock Creek Mine

Andrew Lucas and Tao Hsu

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Blue and orange sapphires from Rock Creek.
Figure 1. This 12.61 ct blue sapphire and 7.90 ct orange sapphire recently mined by Potentate show that large material still comes out of Rock Creek, Montana. Photo by Eric Welch/GIA, courtesy of Potentate Mining.

Potentate Mining marketing director Warren Boyd presented two magnificent sapphires (figure 1) and a suite of fancy sapphires extracted from the company’s Rock Creek operation in Montana in 2016. He also updated the authors on this past mining season.

The larger of the two stones is the 12.61 ct blue sapphire, one of the largest of its kind from Montana. It was faceted from a 6.37 g rough that had been heated. The rough showed strong blue color before heating but had a core with heavy silk inclusions. Heating dissolved the silk and enhanced the blue color. The 7.9 ct orange oval is the largest of this color ever recovered from Montana, to Boyd’s knowledge. This stone was faceted from a 5.2 g rough of pale greenish color with a dark brownish core. While the rough was of very low transparency, the results of heating were better than expected. The heated rough showed a royal blue rim surrounding a bright orange core. Both stones were faceted by Jeff Hapeman (Earth’s Treasury, Westtown, Pennsylvania).

Warren Boyd – Potentate Mining LLC
In this interview, Mr. Boyd delivers an update on Potentate Mining’s production of Montana sapphire and the importance of maintaining consistent levels of production for supply to the jewelry industry. He also provides background on some remarkable sapphires: fashioned 7.90 ct orange and 12.61 ct blue sapphires and a pristine pale blue 59.50 ct crystal, the largest transparent crystal the company has recovered to date.

Most of the Rock Creek sapphire is small, and larger stones with spectacular colors are very rare. Because the bulk of the production comes out of the ground with greenish pastel colors, natural stones with bright colors are also very valuable. Boyd and his clients from Americut showed us a suite of natural fancy sapphires (figure 2) recovered in 2016. The company had produced sapphire rough from its Rock Creek deposit for two consecutive mining seasons and gradually built up a large inventory. Potentate is not planning to increase production but to keep it stable in the near future. The company installed a new water clarifier in its washing plant at Eureka Gulch, which recycles the water used in ore washing. The company will keep its focus on building inventory and bridging the gap between supply and demand.

Colors of Montana sapphire.
Figure 2. As displayed by these rough and cut sapphires, Potentate’s sapphire production includes a wide variety of colors. Photo by Eric Welch/GIA, courtesy of Potentate Mining.

Andrew Lucas is manager of field gemology for education at GIA in Carlsbad, California. Tao Hsu is technical editor of Gems & Gemology.