Opal

Common opal

Besides the diatomaceous earth, the accumulations of diatom shells or tests.

Other varieties of opal

Fire opal, or Girasol, is a translucent to semi-opaque stone that is generally yellow to bright orange and sometimes nearly red and displays pleochroism at certain angles.

Peruvian opal (also called blue opal) is a semi-opaque to opaque blue-green stone found in Peru which is often cut to include the matrix in the more opaque stones. It does not display pleochroism.

Sources of opal

Until the nineteenth century the only source of precious opal known to Europeans was the mining district of Červenica in Slovakia.

Opal without play of color is very common and can be found all over the world, unlike precious opal deposits that are in greater scope found today only in Australia, U.S., Mexico and Ethiopia.

Australia produces around 97% of the world’s opal. 90% is called ‘light opal’ or white and crystal opal. White makes up 60% but not all the opal fields produce white opal; Crystal opal or pure hydrated silica makes up 30%; 8% is black and only 2% is boulder opal.^{[citation needed]}

The town of Coober Pedy in South Australia is a major source of opal. 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 consists of concretions and fracture fillings in a dark siliceous ironstone matrix. It is found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit in the south.^[3]  

Fire opal is found mostly in Mexico and Mesoamerica. In South America opal discovered in 1930 in a city called Pedro II in Brazil is still produced. In Honduras there was also some fine black opal mined from volcanic ash deposits. This opal is known for its stability.^{[citation needed]}

The Virgin Valley Opal Fields Of Humboldt County in Northern Nevada produce a wide variety black, crystal, white, and fire opal.The Fire Opal mined from this locality is designated as the official gemstone of the State of Nevada. Most precious opals are wood replacements. Many specimens have a high water content, and as a result, have a greater tendency to desiccate and crack than most precious opal. Discovered in 1904 the mines are still producing gem materials in large amounts to hundreds of seasonal visitors. Three Fee Dig Mines provide the general public an opportunity to dig the gems themselves. The largest black opal in the Smithsonian Museum, possibly worth in excess of $1 million, comes from the Royal Peacock Opal Mine in the Virgin Valley.

Another 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.

Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil, Honduras, Guatemala, Nicaragua and Ethiopia.

Synthetic opal

As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974 (Klein and Hurlbut, 1985, p.528). The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of color are seen to be arranged in a "lizard skin" or "chicken wire" pattern. Synthetics are further distinguished from naturals by the former's lack of UV light. Synthetics are also generally lower in density and are often highly porous; some may even stick to the tongue.

Two notable producers of synthetic opal are the companies Kyocera and Inamori of Japan. Most so-called synthetics, however, are more correctly termed imitations, as they contain substances not found in natural opal (e.g., plastic stabilizers). The imitation opals seen in vintage jewellery are often "Slocum Stone" consisting of laminated glass with bits of foil interspersed.

The Hamamatsu Photonics Group will utilise the International Space Station's Japanese Experiment Module in 2006 to grow perfect crystalline opals in microgravity over four months, as compared with five million years for naturals. Such opals will be the object of study and elements for optical filters, displays, and data storage.^[4][5]

Local atomic structure of opals

The lattice of spheres of opal that cause the interference with light are several hundred times larger than the fundamental structure of crystalline silica. As a silanols, structures such as Si-O-H, generally form a lesser proportion of the total and can reside near the surface or in defects inside the opal.

The structure of low-pressure polymorphs of anhydrous close-packed layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.  

Microcrystalline opal

Opal-CT has been interpreted as consisting of clusters of stacking of cristobalite and tridymite over very short length scales. The spheres of opal in opal-CT are themselves made up of tiny microcrystalline blades of cristobalite and tridymite. Opal-CT has occasionally been further subdivided in the literature. Water content may be as high as 10 wt%. Lussatite is a synonym.

Opal-C is interpreted as consisting of localized order of α-cristobalite with a lot of stacking disorder. Typical water content is about 1.5wt%. Lussatine is a synonym.

Non-crystalline opal

Two broad categories of non-crystalline opals, sometimes just referred to as opal-A, have been proposed

Opal-AG: Aggregated spheres of silica, with water filling the space in between. Precious opal and potch opal are generally varieties of this, the difference being in the regularity of the sizes of the spheres and their packing.

Opal-AN: Water-containing amorphous silica-glass. Hyalite is a synonym.

Non-crystalline silica in siliceous sediments is reported to gradually transform to opal-CT and then opal-C as a result of sedimentary rocks, as some of the stacking disorder is removed (Cady et al., 1996)PDF (2.03 MiB).

Historical superstitions

In the Middle Ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal. ^[6] However, modern superstition attributes bad luck to the stone, though some believe this is avoided if opal is the owner's birthstone (that is, the owner was born in October) or if the stone is a gift. Even under the last czar at the beginning of the 20th century, it was believed that when a Russian of any sex, of any rank, saw an opal, amongst other goods offered for sale, he or she would not buy anything more, since, in the judgement of subjects of the czar, the opal embodied the evil eye. ^[6] It's possible that the stone's extreme fragility (when compared to other gemstones) has contributed to this bad reputation.

Opals in popular culture

The opal is the official gemstone of South Australia and the Commonwealth of Australia, and the country's women's national team in basketball is nicknamed The Opals.

The official state gem stone for Nevada is precious black Fire Opal, in recognition of the black opal found in Virgin Valley, Humboldt County, Nevada.

Opal is the traditional birthstone of the month of October.

Famous opals

• The Andamooka Opal, presented to Queen Elizabeth II, also known as the Queen's Opal
• The Aurora Australis Opal, considered to be the most valuable black opal
• The Black Prince Opal, originally known as Harlequin Prince
• The Empress of Australia Opal
• The Fire Queen Opal
• The Flame Queen Opal
• The Flamingo Opal
• The Halley's Comet Opal, the world's largest uncut black opal
• The Jupiter Five Opal
• The Olympic Australis Opal, reported to be the largest and most valuable gem opal ever found
• The Pride of Australia Opal, also known as the Red Emperor Opal
• The Red Admiral Opal, also known as the Butterfly Stone

See also

• Optical phenomena
• Upal

References

• Graetsch, H., "Structural Characteristics of opaline and microcrystalline silica minerals", "Silica, physical behavior, geochemistry and materials applications". Reviews in Mineralogy, Vol. 29, 1994. Editors PJ Heaney, CT Prewitt, GV Gibbs, Mineralogical Society of America,
• Klein, Cornelis, and Hurlbut, Cornelius S.; 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7