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At the micro scale precious opal is composed of hexagonal or cubic closely packed
silica spheres some 150 to 300 nm in diameter. These ordered silica spheres
produce the internal colors by causing the interference and diffraction of light
passing through the microstructure of opal (Klein and Hurlbut, 1985, p. 444). 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 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.

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

Australia produces around 97% of the world's opal. 90% is called 'light opal' or
white and crystal opal. White makes up 60% and 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. 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.

Ethiopia is the newest source of high quality Opal gems.

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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.

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.