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.

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 inter growth of species resulting from the un mixing of solid solutions. All of these species, varieties, and inter growths 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.