Of all of the minerals in the Earth's crust, the aluminosilicate, feldspar, is the most common. It is a very useful industrial mineral that finds applications in making glass, plumbing fixtures, tile and pottery. These uses in the United States accounted for about 630,000 ton of U.S. produced feldspar valued at about $27.4 million. It is also a gemstone.
Figure 3. - Oregon sunstone from the Ponderosa Mine.
(Photo is courtesy of Ponderosa Mine.)
Production of gem-quality feldspar in the United States increased rapidly in the past 5 years, declined slightly in 1993, but should continue to increase significantly in the future. Production was valued at more than $0.7 million in 1993. In the United States, gem feldspars are found in igneous, metamorphic, and sedimentary deposits from coast to coast and border to border.
The feldspar group has nine species; there are varieties within some of the species, and there are also intergrowth of species resulting from the unmixing of solid solutions. All of these species, varieties, and intergrowths can be gemstones. Additionally, there are feldspars that have general names because of unique optical phenomena; these are sunstones and moonstones. Fine, gem-quality feldspar is very rare and some species and varieties are more common as gems than others.
New Mexico.--The quality of the adularia moonstone from Catron County is comparable to the best material from India and Sri Lanka. The best of the New Mexico material is water clear, nearly colorless, with only a hint of a soft-tan or brown tone, and has a wonderful blue or silver sheen. Unfortunately, only about 10% of the moonstone has the more desirable blue adularescence. The deposit has produced eye-clean, blue sheen, faceted stones as large as 5 carats, silver sheen faceted stones of almost 10 carats, and good-grade cabochons of 15 to 20 carats. The deposit is not currently being worked, but the author knows of material that is available from work done in the late 1960's.
Oregon.--The premier U.S. gem-quality feldspar is the red labradorite, known as sunstone, mined in Oregon. Large quantities of gem-quality labradorite, most of it water-clear, straw yellow, or yellow sunstone, has been produced from deposits in southeastern Oregon for many years. In sunstone, the labradorite contains millions of copper platelets that reflects the light with varying intensities resulting in a golden-red play of color known as schiller. Although the common color of Oregon sunstone is straw-yellow, it also can be pink, peach, red, salmon red-orange, red-green, and blue-green. It also can be bicolored and tricolored in combinations of yellow, red, and green, and a small percentage is di- and tri-chroic.
In 1988, the availability and quality of sunstone, other than the standard yellow Oregon variety, drastically increased with the discovery of the deposit that was to become the Ponderosa Mine. The vivid-red faceted stones and velvety-red cabochons from this mine set a new world standard for beauty, not just for red sunstone, but for any sun-stone. The opening of the mine also increased the supply of peach, salmon red-orange, red-green, green, blue-green, and bicolored and tricolored in combinations of yellow, red, and green. For the first time ever, there was a continuous U.S. supply of the more desirable colors in most of the calibrated sizes up to several carats needed by the jewelry industry. The largest faceted high-quality red sunstone is over 10 carats. The supply from this and other developed deposits will continue to be adequate for many years into the future.
Virginia.--The red Oregon sunstone is not the only gem feldspar produced in the United States that sets the world standard for quality. Amazonite from Amelia County, Virginia, also falls in this category. The deep, blue-green, translucent amazonite from the Morefield and Rutherford Mines is the finest in the world. The Morefield Mine is currently operated both as a commercial gemstone mine and as a fee-for-dig mine. The Morefield Mine ensures an adequate supply of high quality amazonite for cabochons, beads, carvings, and spheres. High-quality material from the Morefield is available in pieces over 0.5 kilogram in weight. The Rutherford Mine is open only a single weekend per year on a fee-for-dig basis. Colorado, California, and Montana also have deposits of amazonite. Of these locations, the mineral specimen-quality amazonite from Colorado is the best known.
Others.--There is production of various gem feldspars from other deposits in the United States. Small to medium-sized pieces of light brown bytownite from Arizona and New Mexico can be faceted into 0.5- to 2-carat, eye-clean stones. In addition to the Oregon deposits of faceting-grade labradorite, gem-quality material has been recovered and cut from deposits in California and Nevada. Pale-yellow to colorless andesine is recovered from deposits in north-eastern Idaho. The material is found as eroded crystals more than 5 centimeters in length that can cut stones of more than 10 carats. Other States have reported deposits or finds of gem-quality feldspar. Currently, none of these deposits are commercially mined.
The production of gem-quality feldspar in the United States is increasing, as is the use of U.S.
gem feldspar. This increased demand will improve prices for rough and thus provide the incentive
for people to spend the time and money to mine the high-quality deposits in the United States. It is
only a matter of time until additional deposits are mined on a commercial basis.
U.S. Department of the Interior
U.S. Geological Survey
988 National Center
Reston, VA 20192 USA
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Last modification: 11-Jan-2013@16:57