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Origins of Opal..
About 95% of the world's opal comes from Australia. In particular, the town of Coober Pedy in South Australia is a major source. Common, water, jelly, and fire opal are found mostly in Mexico and Mesoamerica. Another Australian town, Lightning Ridge in New South Wales, is the main source of black opal, opal containing a predominantly dark background (dark-gray to blue-black displaying the play of color).
Boulder opal is found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit in the south.
A source of white base opal in the United States is Spencer, Idaho. A high percentage of the opal found there occurs in thin layers. As a result, most of the production goes into the making of doublets and triplets.
The opal is the official gemstone of South Australia and the Commonwealth of Australia. Opal is the official birthstone of the month of October.
The state gem stone for Nevada is precious black opal, which is named for the true black opal found in Virgin Valley, Humboldt County, Nevada. It can also represent the zodiac sign of Gemini.
Precious Opal...
Precious opal shows a variable interplay of internal colours and does have an internal structure. The veins of opal displaying the play of color are often quite thin, and this has given rise to unusual methods of preparing the stone as a gem. An opal doublet is a thin layer of colorful material, backed by a black mineral, such as ironstone, basalt or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch. Given the texture of opals, they can be quite difficult to polish to a reasonable lustre. The triplet cut backs the colored material with a dark backing, and then has a cap of clear quartz (rock crystal) on top, which takes a high polish, and acts as a protective layer for the comparatively delicate opal.
Common Opal...
Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish; resin opal, honey-yellow with a resinous lustre; wood opal, caused by the replacement of the organic material in wood with opal; menilite brown or grey; hyalite, a colorless glass-clear opal sometimes called Muller's Glass; geyserite, (siliceous sinter) deposited around hot springs or geysers; and diatomite or diatomaceous earth, the accumulations of diatom shells or tests.
Opal is a mineraloid gel which is deposited at relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, and basalt.
Opal is one of the mineraloids that can form or replace fossils. The resulting fossils, though not of any extra scientific interest, appeal to collectors.
Opal:
Color: An orangy crystalline glow if put up to light, but when held in hand runs the gamut from opaque white to crystalline orange. All opals have a background color and texture upon which brilliant color plays in ever-shifting patterns are shown, in every color of the rainbow.
Moh’s hardness: 5.5 - 6.5
Refractive Index: 1.44 - 1.47
Countries of Origin: The oldest opal mine was at Czerweniza, in the former Czechoslovakia. There is evidence that this mine was worked in the 14th century, but it may in fact have been in operation centuries earlier-- as the source of stones for Rome. The black opal was first founded in Australia in 1887.
Today Australia produces 85% of the world’s opal. and the distinctive patterns which the color patches make on the picture plane of the stone. Opal is formed in the cavities and cracks of near-surface volcanic rocks. It is also created by percolating water in or near sedimentary volcanic ash that dissolves silica of shells, bones and woods, “fossilizing” them into opal.
In History, Literature and Lore: “Opal” comes from the Sanskrit “upala” and the Latin “opalus,” meaning “precious stone.” The opal’s principal characteristic is its play of beautiful colors. The Romans believed that opal symbolized love and hope; the Arabs, that it fell from the heavens in flashes of lightning. Queen Victoria helped popularize the stone by giving it to her children.
It was a favorite stone of Rene Lalique, the noted artisan of the Art Nouveau movement, who designed opal jewelry for the great actress Sarah Bernhardt. The French believed opal rendered the wearer invisible, making it known, too, as the talisman of thieves and spies. And Australian legend had that the starts are governed by a huge opal that also controls gold in the mines, and human love. The aborigines, however, felt that opal was a devil-- half-serpent, half-human, luring men with evil magic.
What to Look For:
Opals are judged by the range of bright colors which play on the surface in ever-shifting patches on the picture-plane of the stone. The “hot” colors-- such as red, orange or violet-- in the color flashes are the most prized. The stone’s patterns can even have names, such as a harlequin pattern, or rolling flash, or pinfire. Plant and silica-based polymers are occasionally used to improve the appearance of Brazilian, Mexican and Idaho opals.
The beauty of opal helps to outweigh its less than optimal physical properties. These stones are very soft and heat sensitive, and great care should be taken in cleaning. Sudden changes in temperature should also be avoided. As a ring-stone , it should not be worn every day as it can chip. This gem contains water, which may evaporate, leaving it slightly smaller, stressed and cracked.Care should be taken not to purchase opals which have already cracked (or “checked”) from dryness.
Black Opal:
A black background inherent in the rough that offsets the color play of the opalescent material. The background can range from grey to charcoal to matte black; the opalescent material runs the whole gamut of the rainbow.
Country of Origin: Australia.
What to Look For:
Black opal is considered the “king” of opal materials. Its natural dark background greatly enhances the color play of the opalescent material. The blacker the background, the better, with red and orange greatly prized. Buyers should beware that there are some white Australian opals are treated to make them black. After soaking in a sugar solution, they are immersed in sulfuric acid, which carbonizes the sugar and blackens the stone.
Mexican opals, too, have been turned black by smoke treatment in a mixture of charcoal and cow manure. Opal doublets are also manufactured as a lookalike for black opal by sandwiching black onyx and opal.
Fire Opals:
Transparent stone from white(play of color may or may not appear) to translucent yellow, bright orange, red or brick.
Origin: Important deposits are in Mexico (Hidalgo and Queretaro provinces).
What to Look For:
These are the most vibrant of all the orange-colored stones in the gem world, but also the most fragile. They are not suitable for rings but are perfect for pendants. The opalescent types are very beautiful with bright orange background and transparent opalescent patterns throughout the gemstone, and they are the most valuable. Fire opals are the only of the opal materials used for faceting.
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