Let’s Talk Gemstones – Opal Part 2

Australia's most famous opal mines lie on the periphery of this Great Artesian Basin. Mintabie, Coober Pedy, Andamooka, White Cliffs, and Lightning Ridge form an arc along its southern edge. Yowah, Quilpie, and Opalton project on a line north/northwest from Lightning Ridge. Some believe that untold riches, just waiting discovery, lie beneath the unexplored areas of this vast region. However.

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By Edna B. AnthonyMore from this author

The first article on opal described the definitions and provided explanations of the terms and the vocabulary peculiar to this gem species. Australia now stands as the world's major source of opal.

Early in 1997, Fred Ward's fascinating work on this subject was published. For everyone, especially the lay person interested in opal, Fred Ward's book is indispensable. He gives us a guided tour of the opal mines and introduces us to some of the extraordinary people involved in the unique methods of production. He also relates the unusual marketing practices of this beautiful gem. Fred weaves technical information and geological terms into the prose with consummate skill, and he illustrates with such clarity that one is hardly aware of having grasped sophisticated concepts. AND THE PHOTOGRAPHY IS SUPERB!

More than half of the eastern sector of Australia lay under the waters of the Great Artesian Basin during the Jurassic and the Cretaeceous Periods. As the sediments accumulated, many layers of sandstone, shales, and limestone were deposited. After the recession of the sea, the vast area became a desert. Tiny spheres of silica from saturated solutions seeping through these weathering deposits precipitated into cavities, sometimes replacing shell, wood, and bone. A regular alignment of these spheres, having a uniform size and shape caused by the constant slow rate of deposition and evaporation, produced precious opal. A variation in the size, shape, or alignment of these spheres resulted in the formation of common opal.

Australia's most famous opal mines lie on the periphery of this Great Artesian Basin. Mintabie, Coober Pedy, Andamooka, White Cliffs, and Lightning Ridge form an arc along its southern edge. Yowah, Quilpie, and Opalton project on a line north/northwest from Lightning Ridge. Some believe that untold riches, just waiting discovery, lie beneath the unexplored areas of this vast region. However, Dr. Joel Arem, (in the context that "worldwide demand is putting tremendous pressure on opal prices") states in his COLOR ENCYCLOPEDIA OF GEMSTONES that "opal deposits have been worked so intensely that they are becoming depleted" and that "new discoveries are rare."

Mintabie, not as well known as other mines, is unique for its good quality light, dark, and black opal. Sometimes, all of these colors are found together in the same pocket. About twenty percent of the production from Mintabie is black opal. Opal that is free of its matrix-sandstone is also recovered from there.

Some underground mining for opal does take place. However, the random distribution of opal material to a depth of 100 feet in hard sandstone dictates the exploration of an entire claim. Open cut or strip mining is the method used most frequently. Since restoration of "Precious Stone Fields" is not required by the Australian government, miners are free to walk away from the devastated land. Such shredded earth from exhausted claims has created a surreal moonscape on the abandoned area.

Horizontal seams of light opal, as well as some crystal opal, deposited in the soft clays of Coober Pedy make extraction by tunnel machines economically feasible. Blowers bring the material to the surface for further processing and sorting.

Coober Pedy was exploited in the 1960s by an American, George Manning. He had large quantities of light-colored opal material cut into calibrated cabochons in Hong Kong, then shipped them to eager buyers in America. Until recently, his was the opal most familiar to Americans.

Harsh conditions prevail at most opal mines. Some of the residents have constructed luxurious homes underground, especially at Coober Pedy, to make life more comfortable in these isolated sites. Tourism is now a secondary economic factor at Coober Pedy. Its residents enjoy urban amenities in a number of buildings erected above ground.

Andamooka remains a typical dusty "wild west" desert town. It was the world's largest producer of light-colored opal until the mid 1980s. Tunneling was the desired method of production during its most active period. Its famous vitreous transparent crystal opal is rivaled in beauty only by the more unstable variety found in Virgin Valley, Nevada.

The crystal opal produced in Oregon and Idaho, though of gem quality, is not comparable. The opal mined at Andamooka is considered by some to be the most stable in the world, because of its very low water content. Unique Andamooka opal matrix is often "smoked" or "sugar treated" to resemble black opal. The porous matrix absorbs sufficient carbon released by the processes to both darken its body tone and enhance its play of color.

Fred Ward describes this type of opal as having "a black peppery appearance with a speckled play of color." This particular characteristic, along with its lighter weight, distinguishes this opal from black opal.

White Cliffs is the only place where the marvelous and very rare psuedomorphic "pineapple" opal has been found. The pineapple opal formed when a mineral crystal of a specie, now believed to have been ikalite rather than glauberite, was first replaced by calcite and then by opal. Despite their rarity, most pineapple opals have been destroyed by gem cutters, who were able to profit more from the opals cut from the psuedomorphs than from having the single pineapple opal specimen.

In his book The Story Of Gems, Herbert P. Whitlock, a former curator of Minerals and Gems of the American Museum of Natural History likens the light opal produced at White Cliffs to that found in Hungary, but with "broader flashes of color" and in "masses capable of furnishing larger stones." This text is in direct contrast to the statement by Dr. Joel Arem in Color Encyclopedia Of Gemstones that "the opal is usually small, with veinlets of precious opal within common opal."

Lightning Ridge, a "free-wheeling" town of about fifteen thousand people, now stands as the major source of the world's finest black opal. Here, the black opal is recovered from seams often more than forty feet below the surface. Heavy equipment is lowered through shafts, assembled below ground, and then used to work the seams in a fraction of the time it took to mine a claim with hand digging. The material is raised to the surface and washed in "co-op" agitators. This method permits faster and more economical sorting and allows easier identification of promising opal material.

Lightning Ridge maintains its own cutting center to retain control of the gems and maximize profits. Buyers deal with individual owners, who sell most of the best gems to customers from Japan and Asia. Sadly, less than eight percent of the finest opal reaches the United States.

Northwest of Lightning Ridge stand the ironstone formations of Queensland, the source of the brilliant boulder opals. These rock formations extend from and include the areas of Yowah, Quilpie and Opalton. A small area around Yowah yields the unique and very rare opal in matrix known as Yowah nuts. These expensive specimens are usually available to collectors only at gem shows and through auctions.

The Yowah nuts were once hollow ironstone concretions about the size of a walnut, and sometimes these contained brilliant opal cores. Some lie on or near the surface of the surrounding sandstone, but miners often use scraping equipment to expose the concretions. They may retain the name Yowah nuts only if a sizeable recognizable portion of the shell remains. If only a small portion of the shell is present, then they are properly called boulder opal. Complete removal of the shell changes the classification to solid opal. At Quilpie and Opalton, sandstone opal, which forms unattached to matrix and seams of boulder opal, lie within the surrounding sandstone.

When the opal is distributed throughout the ironstone matrix in a form not suitable for recovery, the material is often used for unusual decorator objects. It wasn't until the 1960s that heavy equipment and saws capable of handling the tough ironstone made extensive commercial development in the fields feasible. In the last few years, boulder opal has become well known and appreciated by gem enthusiasts all over the world. Its toughness, the brilliant colors, and a freeform style make it especially appealing to designers of unique fine jewelry.Opal mining in Australia is probably the least regulated major industry in the world. For the most part, it is a rough and tumble, cash and carry, cards close to your vest, buyer beware, and a "you'd better know your opals" business!

While Australia overshadows all other countries in the production of most varieties of opal, Mexico produces fire opal that exhibits a special "it" quality almost always referred to as Mexican fire opal. This gem can show a play of color, but it is the body that color makes it so distinctive. The state of Queretaro is the major source today of Mexican opal.

Fire opal is found in the cavities of volcanic lava flows in Central and South America, as well as in Idaho, Oregon, and Nevada in the United States. Hand dug pits in Mexico still dominate the method of production there. It is a relativity inexpensive gemstone, and one that can have special faults. Opal formed in volcanic environments often crazes and cracks more frequently than that found in sedimentary deposits. The opal can also fade. Rhyolite spheres called "thunder eggs" sometimes contain such opal. It is interesting to note that much of the Mexican opal will craze within a period of less than an hour, although sometimes many months pass before crazing occurs.

Two new varieties have been recovered in Mexico recently. Opal deposited in rhyolite matrix, cut to retain some of its red, tan, pink, and cream-colored matrix, can superficially resemble Australian boulder opal. The new Leopard opal made its appearance at the gem and mineral show in Tucson in 1996. It is recovered from vesicular basalt formations, where the vesicules were filled with light-colored opal. The play of color spots do remind one of a leopard skin.

A lovely blue translucent common opal called Andean or Peruvian opal is found in the Andes mountains near San Patricio, Peru. Copper may be the essential trace element that causes its soft distinctive color. It has been used by native South Americans for more than a thousand years. Recent commercial production is making more of this inexpensive material available to carvers and jewelry designers at mineral, gem, and jewelry shows. Sometimes, this aqua blue opal can dry out and lose its clarity.

There are many other lesser known sources of opal. Honduras produces a light-colored opal in a dark reddish to black matrix. Prase opal, colored by nickel, is found in Poland. Much of the Indonesian opal material resembles the water or jelly opal found in Mexico.

No two opals are ever identical. Opal is generally a soft and fragile gemstone that requires proper care to preserve its great beauty. Before purchasing any expensive gemstone, become familiar with all the proper methods of setting and caring for such a gem. This is especially true of the unique and very beautiful opal.

Opal Gemstone Properties

Varieties:refer to Jan/Feb 1997 Issue of NMFG newsletter
Phenomena:play of color and girasol effect
Crystal Systemamorphous
Habit:layers, veins, nodules, and psuedomorphs
Fracture:conchoidal and brittle
Fracture Lustre:
Lustre:vitreous, waxy, and pearly
Specific Gravityvariable 1.98 to 2.25
Hardness5.5 to 6.5
Refractive Indexvariable 1.44 to 1.47; Mexican opal as low as 1.37, usually 1.42 to 1.43
Optic Characterisotropic
Dispersion:very low
Pleochroismnone (please disregard info printed here Jan/Feb issue)
Ultraviolet  FluorescenceVariable. Strong white, medium blue, dull white, bright blue, pale yellow, brownish bright green (indicates U minerals). Fire opal often greenish brown. Black opal usually inert. Common opal often green. Phosphorescence sometimes strong.
Color Filterno information
Solubilityetched by HCL
Thermal TraitsVERY SENSITIVE TO HEAT and sudden temperature changes
Treatmentsdyes, sugar cooking, and smoking
Related Links:

Let's talk Gemstones - Staurolite
Let's Talk Gemstones - Peridot
Let's Talk Gemstones - Zircon Group

By Edna B. Anthony, Gemologist
All rights reserved internationally. Copyright the New Mexico Faceters Guild © 2002 Users have permission to download this information and share it as long as no money is made. No commercial use of this information is allowed without written permission from the New Mexico Faceters Guild
The purpose of the New Mexico Faceters Guild (NMFG) is to bring together persons who are interested in faceting or faceted stones. We promote the art and science of faceting and provide a means of education and improvement in faceting skills. Finally, we provide a means of communication between those persons involved or interested in faceting as a hobby.
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Edna B. Anthony

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