Gem News International Gems & Gemology, Spring 2020, Vol. 56, No. 1

Interesting Sapphires from Montana and Australia

Bicolor sapphires from Rock Creek, Montana.
Figure 1. Bicolor sapphires from the Rock Creek mine, Montana. Cutters took advantage of the color inhomogeneity of the rough to create these interesting color combinations. These stones range from 1.61 to 7.36 ct. Photo by Emily Lane; courtesy of Potentate Mining.

While sapphire has always been popular at trade shows, occasionally the authors find some interesting stones with attractive color distribution or growth patterns that have a niche in the market.

At this year’s AGTA show, Potentate Mining carried some spectacular bicolor faceted sapphires and a variety of slabs from its Rock Creek mine in Montana. The Rock Creek operation is one of four active sapphire mines in Montana. Stones from this deposit tend to be flat, showing pastel colors, and many stones show bicolor or have an orange or yellow core called “yolk” by the local miners. Gem cutters and jewelry designers have been finding innovative ways to take advantage of these challenging attributes. Through careful design and cutting, some stones show attractive color combinations such as yellow/green, yellow/light blue, orange/blue, or simply different shades of yellow (figure 1). This type of bicolor is very characteristic of Rock Creek sapphires.

Flat rough sapphires from Rock Creek, Montana.
Figure 2. Flat rough sapphires from Rock Creek attract jewelry designers and cutters. The slabs are grouped and sold by their weight. Photo by Tao Hsu.

As for the flat rough, Potentate offers several categories based on color, pattern, and weight (figure 2). According to the company’s marketing director, Warren Boyd, both cutters and designers have shown strong interest in these slabs, especially those weighing half a gram and above. Designers seem to prefer slabs with patterns while cutters or carvers prefer slabs with solid colors to make special cuts.

Blue sapphire thin slabs from Inverell, Australia.
Figure 3. This pair of blue sapphire thin slabs displays a very attractive color. They were sliced off a dark blue sapphire crystal from Inverell, Australia. Mr. Coldham also offered thin slabs of blue and parti-colored sapphires from Australia. Photo by Tao Hsu.

At the GJX show, Terry Coldham from Intogems showed the authors a pair of thin blue sapphire slabs with characteristic hexagonal growth patterns. The pair in figure 3 is from the Inverell sapphire field of New South Wales, Australia. Blue sapphires from this area tend to show a very saturated and dark inky blue color. Slicing them into thin slabs makes the blue color much lighter and more desirable. However, the size and quality of this pair are extremely rare to find. Mr. Coldham also carries many parti-colored and fancy-color Australian sapphires (see figure 4 in the Tucson Overview), which also sold well at the show.

Watch videos of the sapphire slabs from Montana and Australia:

Sapphire slabs from Montana and Australia

Tao Hsu and Jennifer Stone-Sundberg are technical editors for Gems & Gemology.

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