opal
SiO
Amorphous
Environment
In gas holes in fresh volcanics, deposits from hot springs, and in sediments, as poikilitic units enclosing sand or replacing fossils.
Crystal description
There are two types, common opal and precious opal. Common opal is amorphous, therefore not in crystals except as pseudomorphs of others. Can be colorless (hyalite) or colored blue, green, red, or yellow by impurities; in amygdules, veins, and seams; also botryoidal, reniform, stalactitic. Commonly pseudomorphous after wood, shells, or bone. Precious opal is transparent material with a play of color. Electron microscope photographs of precious opal reveal a regimented structure that explains the fire. Silica, having flocculated into tiny, uniformly sized and perfectly aligned spheres, creates a microgrooved effect, like that on a compact disc, and the rulings in a diffraction grating, to create a color play of reflected light.
Physical properties
Colorless or with all light tints; also with rainbow play of color. Luster glassy to resinous; hardness 5-6; specific gravity 1.9-2.2; fracture conchoidal. Rather fragile and brittle; tending to lose water and crack; transparent to translucent; often highly fluorescent (yellow-green), from uranium impurities.
Composition
Silicon dioxide, like quartz, but with water to 10%.
Tests
Infusible and insoluble, but gives water in closed tube upon intense ignition; usually decrepitates in flame, may whiten. Distinguished from chalcedony by the shiny fracture surface.
Distinguishing characteristics
Broken fragment might be confused with quartz, but opal's lesser hardness is a good guide. Fluorescence frequent (when uranium is present).
Occurrence
One of the varieties of opal, the variety characterized by a play of rainbow colors in what is essentially clear material, is known as precious opal and is a valuable jewelry stone. Fire opal is the name given red-orange transparent common opal, often faceted and associated with precious opal in Mexico. Common opal has no particular value, though it is often highly fluorescent and collected for that reason.
There are many occurrences of opal deposited from hot water, as in the hot spring terraces and steaming geyser chimneys of Yellowstone National Park. A light yellow altered wood occurs in Virgin Valley, Nevada, together with precious opal log replacements. Diatomaceous earth is made from the fossil external skeletons of microscopic plants. Precious opal was found in volcanic rocks in the first known occurrence, in Czechoslovakia, and later in Idaho and California, and preeminently in the Querétaro district of Mexico. Of greater value, however, are the opals of the sedimentary rocks of e. Australia, which at many different places yield numerous types of precious opal. It is found in concretions and in cracks and crevices in soft sediments, and often in Queensland as opalized fossils. It occurs on seams in clay-iron stone boulder opal. An opal-cemented white quartzite from Australia is commonly dyed to resemble black opal. Similar material has lately been found at some depth in Louisiana. There is an occurrence of a fairly hard white precious opal in Brazil, at Dom Pedro II, Ceará. Colorless to blue hyalite is common on seams in pegmatites in the Spruce Pine district, North Carolina. Glassy hyalite droplets in Oregon and Mexican rhyolite pockets resemble masses of frog eggs, as do oolitic opal concretions around a hot spring in Japan, each enclosing a tiny central speck.

The noun opal has one meaning:
Meaning: a translucent mineral consisting of hydrated silica of variable color; some varieties are used as gemstones

opal

The mineraloid opal is amorphous SiO₂·nH₂O; hydrated silicon dioxide, the water content sometimes being as high as 20%. Opal ranges from colorless through white, milky blue, gray, red, yellow, green, brown and black. Often many of these colors can be seen at once, caused by interference and diffraction of light passing through minute, regularly arranged apertures within the microstructure of opal, known as Bragg's lattice. These apertures are 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 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. The word opal comes from the Sanskrit upala, the Greek opallios, and the Latin opalus, meaning "precious stone." 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.


Boulder opal carving of a walrus, showing flashes of colour from the exposed opal. The carving is 9 cm (3.5 inches) long.

Sources 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 has a main source in Quilpie, Queensland. The main source of opal in the United States is Spencer, Idaho. The opal is the official gemstone of South Australia. Opal is the official birthstone of the month of October.

Synthetic opal

As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of colour 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 Gilson opals often seen in vintage jewellery are actually an imitation consisting of laminated glass with bits of foil interspersed.

See also

• Isabella Rigney
• List of minerals
• Optical phenomena