In the last issue, we reviewed some of the changing fortunes of sapphire in the world gem market. In this issue, I will review the history of American sapphire mining, in essence, Montana sapphire mining. Although a few noteworthy sapphires have been found in North Carolina’s Macon County, the most recent discoveries from there showed small sapphires of low to poor quality (Arem, 1987). When American sapphire mining is discussed, it is usually assumed that the source is Montana.
Montana sapphires originate from two distinctly different types of geologic locations. The first type of deposit lies in hard rock dikes, the source of Yogo Gulch stones. The second type of locality is alluvial deposits, where water-worn pebbles of sapphire are sifted from the gravel banks of ancient rivers, now lying adjacent to the prevailing river beds.
Sapphire was first discovered in Montana by gold miners who noticed how certain small pebbles tended to accumulate at the bottom of their sluices where the gold would normally collect. These pebbles were deemed a nuisance, because they cluttered the sluice box. The gold miners usually removed and discarded the pebbles. Although colorful, these river-worn pebbles were first noticed in 1865. However, most of the pebbles showed a low color saturation.
It was in 1895 that gold prospector Jake Hoover decided to collect a few superb blue stones in lower Yogo Creek. He recognized that they might be valuable gemstones and sent a sample to an assay office. The stones were eventually forwarded to George F. Kunz at Tiffany and Co. Kunz, America's first gemologist, positively identified the stones as sapphire. He sent a handsome check to Jake Hoover of $3,750.00, which amounted to more than Hoover’s current gold operations. Kunz requested that more of these sapphires be shipped to him, and Tiffany marketed the stones. Hence, American sapphire mining was born (this brief history courtesy Keith Mychaluk, 1995).
Yogo Gulch rates historically as the best known location for sapphire.
The stones from there are clean and uniformly intense in a color termed
“cornflower blue”. Some describe Yogo sapphires as royal American sapphires.
Sapphires from Yogo do not require heat-treatment and generally show good
clarity. Yogo stones tend to be small, usually below one carat in the rough.
Yogo sapphire rough larger than two carats is very rare.
Yogo sapphire production is low because of the difficulty in extracting the sapphires from the hard rock. Blasting can destroy the sapphire, and weathering of the rough from the hard rock requires time. As a result, Yogo sapphires are relatively expensive when compared to other Montana sapphires. Yogo Gulch is one of very few areas world wide where sapphire is extracted directly from the host rock. Mining at Yogo is an expensive and tedious process, with relatively few sapphires produced. Reports estimate that 2.5 million carats of rough have been recovered in the 100 years of operations at Yogo Gulch (Mychaluk, 1995). The numerous efforts to commercially mine this area have been financially devastating to many entrepreneurs.
Alluvial mining in Montana has centered around a few areas, most notably near Helena and Phillipsburg. Commercial production efforts have waxed and waned over the years, as have the small operations that allowed individual picking through washed gravel for a fee. Gravel with sapphire is still available at gem shows in bags weighing several pounds. These bags of concentrate yield rather bland pebbles, usually under one carat in the rough.
The recent commercial activity has centered around Eldorado Bar, Dry Cottonwood Creek, and Gem Mountain. For years, miners have asserted that there were millions of carats of gem quality sapphire in these deposits, yet little was seen until recently. Some of the mining efforts have been costly and not all that commercially feasible.
The aluminum mining conglomerate, AMAX was active in efforts to commercially mine sapphire during the early 1990's. They eventually stated that their geological studies were not very encouraging, that “Gem mining is too small to warrant taking the management time away from their well-defined coal and copper projects” (Verbin, 1994). They did not renew their lease on their sapphire mining claims in Montana.
Despite the pessimism expressed by AMAX, others have maintained their enthusiasm for the commercial potential of the area. Over the past few years, significant amounts of gem sapphire have been recovered. American Gem Corporation reported recovering 1.8 million carats in 1995, and Gem River reported 1.4 million carats from their operation (Lurie, 1996). (You will note that the combined total for those two mining operations for 1995 alone is more than the life-time total recovered at Yogo.)
American Gem Corp. has been acquiring claims in the Montana area with the hope to become a major player in the world gem market and to be known for their distribution of Montana sapphire. They now have control of over 110 square miles of potential sapphire deposits. Yields vary from about 44 to 225 carats per cubic meter to over 1,000 carats per cubic meter in selected areas. They estimate over 50 million carats of reserves in their three claims alone (Koivula et al., 1994).
Alluvial deposit sapphire varies in color and quality. Some samples
are largely green, pale blue, and steely grey. Others contain a phenomenal
range of intense colors. These stones are much larger than Yogo stones.
Alluvial deposits yield stones of commercial quality that often are sorted
to select parcels of three to ten carat stones. The stones are often less
than 6 mm. in diameter, but significant quantities are recovered in the
6 mm. to 12 mm. sizes. Stones larger than 12 mm. remain rare, even from
the alluvial deposits (Austin, 1993).
I have seen various material from Montana, both “mine-run” and after heat treatment. One of the problems encountered is that mine-run material is often cloudy. This problem is significantly resolved by proper heat treatment. While some sapphire material is not so impressive, other rough parcels can be absolutely mouth watering from the intense and vibrant colors displayed. The lack of consistent and effective treatment of this material has been one of the stumbling blocks that have proved fatal to the past efforts of commercially mining Montana sapphires.
Recent technological developments seem to have changed all that. Now, a much greater quantity of clean faceted stones are produced when compared to the earlier efforts. An excellent article on this enhancement process was written by John Emmett and Troy Douthit (1993). They describe their experiences in treating over 75,000 stones in their sapphire treating business, Crystal Research (subsequently acquired by American Gem Corp.).
Heat-treatment of sapphire has been done in the Orient for many years with primitive methods, such as charcoal ovens. The results have been variable, especially when those techniques were used on Montana material. Pure corundum is colorless and composed of aluminum and oxygen. The coloration of sapphire, especially in the blue colors, stems from the intervalence charge transfer of iron and titanium atoms. The relationship between the iron and the titanium atoms with the electrons in the corundum molecule is such that the iron is usually distributed throughout the sapphire crystal lattice. The titanium, however, is often bound with rutile needles inside the sapphire crystal.
This composition prevents the intervalence charge transfer, and, subsequently, prevents the desired blue color. If the titanium can be freed from the rutile, the color will change towards blue. This is accomplished by heating the crystals to 1,600 degrees C., sufficient to dissolve the rutile needles but not so high as to melt the corundum (2,052 degrees C.). I have run across heated sapphire with the crystals fused together, apparently from over-zealous heating.
Heat alone is not enough. Attention also must be given to the atmosphere inside the heating oven. A reducing atmosphere rich in hydrogen is needed to facilitate the process. The authors, mentioned above, describe that special atmosphere requirement when working with pale blue stones, pale green stones, and near colorless stones. Approximately two-thirds of the stones slated for treatment can be converted to well-saturated shades of blues and yellows.
The color change is permanent, but such a treatment must be disclosed.
I understand that we should assume all sapphires have been heat-treated,
unless claimed otherwise. I would be suspicious of such claims, unless
a clear possession of the material from the time it had been mined can
be established (like a pedigree).
Another good review of the recent developments in heat treatment of sapphire was published in the December 1996 Issue of Lapidary Journal, where an article by Si and Ann Frazier summarized the report by Emmett and Douthit.
It remains to be seen what marketing efforts will ensue, and how they will change the reputation and the acceptance of this lovely gem material. Is the advertising hype backed by the ability to truly follow through, or will the bluff be called on Montana's sapphire reputation?
In the next issue: Poised on the brink?
Arem, Joel (1987) Color Encyclopedia of Gemstones (second edition); New York; Van Nostrand Reinhold.
Austin, G. (1993). “The changing color of U.S. sapphire.” Colored Stone; Volume 6, #1, pp. 370-371.
Koivula, John., Kammerling, Robert., Fritsch, E., Johnson, M., and DeGhionno, D. (1994). “Gem news: Update on Montana sapphires.” Gems & Gemology; Volume 30, #4 of the 1994 Winter Issue; p. 276.
Lurie, Mark (1996). “Will new sources satisfy sapphire demand?” Colored Stone; Volume 9, #2, pp. 1, 38-39.
Mychaluk, Keith (1995). “The Yogo Sapphire Deposit.” Gems & Gemology; Volume 31, #1 of the 1995 Spring Issue, pp. 28-41.
Verbin, E. (1994). “Montana miners play musical chairs.” Colored Stone; Volume 7, #2, pp. 1, 6, 39.
Ward, Fred (1992). Rubies and Sapphires; Bethesda, Maryland; Gem Book Publishers.
Voynik, Stephen (1985) Yogo-The Great American Sapphire; Missoula, Montana; Mountain Press Publishing.
Ward, Fred; Rubies and Sapphire; National Geographic; October 1991.