Thursday, October 30, 2008
Opal is brittle, heat sensitive, and breaks and scratches easily; additionally, some varieties self- destruct through the loss of water. Even with these drawbacks, opal's unsurpassed beauty guarantees its status as a premier gemstone. The derivation of its name even adds to its position. Reportedly, opal's name evolved from the Roman word opalus from the Greek word opallios meaning "to see a change of color." The Greek word was a modification of the ancient Indian Sanskrit name for opal, upala, which meant "precious stone." If one spoke in mixed tongues, then opal would be opallios upala, "to see a change of color precious stone."
As indicated by the derivation of its name, opal has centuries of history as a treasured gemstone. Historically, beliefs associated with the wearing of opal have varied. The early Greeks thought that opals gave their owners the powers of foresight and prophecy, and the Romans adored it as a token of hope and purity. Eastern people regarded it as sacred, and Arabs believed it fell from heaven. In the nineteenth century, superstitions grew about the bad luck or fate that could befall one for wearing opal if it were not the wearer's birthstone. Today, these superstitions have diminished, but some people still believe it is bad luck to wear opals. Opal has over one hundred variety and trade names, but the list of accepted or commonly used names is much shorter. The most important and most widely known opal is the precious opal. Precious opal may be subdivided further by color modifiers, white, black, pinks, and blue, which describe the body color of the opal. Australia is famous for its white and black precious opal. Fire opal, the bright red, reddish-yellow, orange, or brownish-red body colored opal is the second most important opal commercially. Until recently, the best fire opal came from Mexico.
Arizona.--Two commercial mining operations in Arizona produce blue precious opal. The body color is a light or pale blue with strong play of color in red, blue, green, and orange. The two operations sell most of its material as finished stones at the Tucson Gem and Mineral show and other local gem shows.
Idaho.--In Idaho, opal is the second largest contributor to the total value of gem material produced. The varieties produced include precious (white and pink), yellow, blue, pink, and common. The Spencer opal mine, the largest privately owned gem stone producer in the State, is the major producer of opal. At Spencer the precious opal occurs as one or more thin layers within common opal partially filling gas cavities within a rhyolite-obsidian flow. About 10% of the material is thick enough to cut into solid gems; the remainder is fashioned into doublets and triplets. The Spencer Mine is the source of pink common opal and pink precious opal.
Louisiana.--The reported precious opal from Louisiana is a sandstone/quartzite with precious opal cement and matrix. It has blue or purple play of color. The material could be cut into cabochons for jewelry and other items of interest. To date, most of the material has been cut into large (over 2-inches in diameter) gemstone spheres.
Nevada.--Nevada is known for precious opal from Virgin Valley. The first discovery of precious opal in the Virgin Valley area was in 1905 or 1906. Since then a significant quantity of the highly prized opal has been recovered. Virgin Valley opal is comparable to any in the world for its vivid play of color and in terms of the size of material available. Individual pieces weighing over 3 kilograms have been recovered from the Virgin Valley deposits. In 1993, miners found a 100-kilogram opalized log containing precious opal. The material varies in body color from deep pure black to brown to yellowish-white to white to colorless. The play of color includes all colors common to precious opal, red, blue, green, yellow, orange, etc. The opal occurs primarily as replacement of wood, or sometimes, as replacement of conifer cones. Some opal does occur as nodules filling void spaces in clay. The wood replacement is so complete, that generally the wood grain and banding are no longer visible. The exception to this would be that often the exterior wood texture is still present as a brown or black rind.
The uses of the opal can be restricted because of crazing. Crazing is the breakdown or deterioration of opal by the development of very fine cracks all over the surface that extend until they intersect. In the worst cases, the surface of the opal deteriorates into a crumbling sand-like material. Because of the crazing the opal is not well suited for use in jewelry, but displayed in water, glycerine, mineral oil or other liquids makes remarkably beautiful mineral specimens. The mineral collections of most of the better museums contain very fine pieces of Virgin Valley opal. Many museum pieces are crazed from exposure to the air. During the summer months, at least two dig for fee mines in Virgin Valley are open to individuals. One mine is operated by the Hodson family and the other by the Wilson family. Individuals pay a daily fee to dig and keep all the opal that they find. Other mines in the area are commercial opal producers.
Two other locations in Nevada also have produced precious opal. The opal does not have as good a play of color as that from Virgin Valley and it has the same crazing problem. One location is south of the Virgin Valley in the Calico Mountains of Humboldt County, and the other is near Gabbs in Nye County. The three precious opal locations and many other locations around the State produce common opal and opalized wood.
Oregon.--During 1988, West Coast Gemstones, Inc., began mining and marketing a variety of very fine-quality opals from Opal Butte in Morrow County, OR. The varieties produced includes hyalite, rainbow, contra luz, hydrophane, crystal, fire, blue, and dendritic. Exquisite stones as large as 315 carats have been cut from contra luz rough from this deposit.
The deposit at Opal Butte has been known since the late 1800's. It was of interest only to hobbyists until recently because people believed most of the opal was unstable. Stability can be a problem with the opal, crazing can occur when stresses are created from shrinkage due to water loss. West Coast has developed methods of drying the opal that greatly reduce crazing. Even with the drying procedures, the stability varies from 20% to 90% depending upon the variety.
The opals are found in rhyolite geodes (thundereggs) in a perlite that has altered to a pastel colored clay. The geodes that contain gem-quality opal are only about 10% of the total geodes mined and only about 1% of the geodes contain gem-quality opal with play of color. The remaining geodes contain agate, quartz crystals, or common opal. The geodes vary greatly in size, from a few centimeters in diameter to over a meter.
Monday, October 20, 2008
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.
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.





