Opal
Opal is a mineraloid gel which is deposited at a 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.
The water content is usually between three and ten percent, but can be as high as 20%. Opal ranges from clear through white, gray, red, orange, yellow, green, shore, blue, magenta, rose, pink, slate, olive, brown, and black.  Of these hues, the reds against black are the most rare and dear, whereas white and greens are the most common; these are a function of growth size into the red and infrared wavelengthssee precious opal.  Common opal is truly amorphous, but precious opal does have a structural element. The word opal comes from the Latin opalus, by Greek opallios, and is from the same root as Sanskrit upala for "stone", originally a millstone with upara for slab. (see Upal). Opals are also Australia's national gemstone.
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.

# Precious opal
Precious opal shows a variable interplay of internal colors and does have an internal structure. At the micro scale precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic closed-packed lattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of opal. It is the regularity of the sizes of the spheres, and of the packing of these spheres that determines the quality of precious opal. Where the distance between the regularly packed planes of spheres is approximately half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The spacing between the planes and the orientation of planes with respect to the incident light determines the colors observed. The process can be described by Bragg's Law of diffraction.
Visible light of diffracted wavelengths cannot pass through large thicknesses of the opal. This is the basis of the optical band gap in a photonic crystal, of which opal is the best known natural example.
In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed play of color. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal. Potch does not show a play of color.
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 opal, backed by a swart 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.
Combined with modern technique of polishing, doublet opal produces similar effect of black or boulder opals at a mere fraction of the price. Doublet opal also has the added benefit of having genuine opal as the top visible and touchable layer, unlike triplet opals.
The triplet cut backs the colored material with a dark backing, and then has a domed cap of clear quartz (rock crystal) or plastic on top, which takes a high polish, and acts as a protective layer for the relatively fragile opal. The top layer also acts as a magniflier, to emphasis the play of colour of the opal beneath, which are often of a lower quality. Triplet opals therefore have a more artificial feel to it and are not classed as precious 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 (which can sometimes be of gemstone quality); resin opal, honey-yellow with a resinous luster; 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.

# Other varieties of opal
Fire opal  Fire opals are transparent to translucent opals with warm body colors yellow, orange, orange-yellow or red and they do not show any play-of-color. The most famous source of fire opals is the state of Queretaro in Mexico and these opals are commonly called Mexican fire opals.
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
Australia produces around 97% of the worlds 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.
The town of Coober Pedy in South Australia is a major source of opal. Andamooka in South Australia is also a major producer of matrix opal, crystal opal, and black 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.
The Virgin Valley opal fields of Humboldt County in northern Nevada produce a wide variety of 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. The largest black opal in the Smithsonian Museum 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.
In late 2008, NASA announced that it had discovered opal deposits on Mars.

# 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. 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 fluorescence under 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.

# 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 mineraloid, there is no unit cell that describes the structure of opal. Nevertheless, opals can be roughly divided into those that show no signs of crystalline orderi.e., amorphous opaland those that show signs of the beginning of crystalline order, commonly termed cryptocrystalline or microcrystalline opal. Dehydration experiments and infrared spectroscopy have shown that most of the H2O in the formula of SiO2nH2O of opals is present in the familiar form of clusters of molecular water. Isolated water molecules, and 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 silica  consist of frameworks of fully-corner bonded tetrahedra of SiO4. The higher temperature polymorphs of silica cristobalite and tridymite are frequently the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline opals also appear to be closer to that of cristobalite and tridymite than to quartz. The structures of tridymite and cristobalite are closely related and can be described as hexagonal and cubic 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 \alpha-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  diagenesis, due to the increasing overburden pressure in sedimentary rocks, as some of the stacking disorder is removed.

# 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. 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 tzar, the opal embodied the evil eye. 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, The Commonwealth of Australia and 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 uncut 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