is a hydrated amorphous form of silica (SiO2·nH2O); its water content
may range from 3 to 21% by weight, but is usually between 6 and 10%.
Because of its amorphous character, it is classed as a mineraloid,
unlike crystalline forms of silica, which are classed as minerals. It 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, marl, and basalt. Opal is the national gemstone of
The internal structure of precious opal makes it diffract light;
depending on the conditions in which it formed, it can take on many
colors. Precious opal ranges from clear through white, gray, red,
orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown,
and black. Of these hues, the black opals are the most rare, whereas
white and greens are the most common. It varies in optical density from
opaque to semitransparent.
opal shows a variable interplay of internal colors, and though it is a
mineraloid, it has an internal structure. At microscopic scales,
precious opal is composed of silica spheres some 150 to 300 nm in
diameter in a hexagonal or cubic close-packed lattice. It was shown by
J. V. Sanders in the mid-1960s, that these ordered silica spheres
produce the internal colors by causing the interference and diffraction
of light passing through the microstructure of the opal. The regularity
of the sizes and the packing of these spheres determines the quality of
precious opal. Where the distance between the regularly packed planes of
spheres is around 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 colors that are observed are
determined by the spacing between the planes and the orientation of
planes with respect to the incident light. 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. The notion that opals are photonic crystals for
visible light was expressed in 1995 by Vasily Astratov's group. 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.
For gemstone use, most opal is cut and polished to form a cabochon.
"Solid" opal refers to polished stones consisting wholly of precious
opal. Opals too thin to produce a "solid" may be combined with other
materials to form attractive gems. An opal doublet consists of a
relatively thin layer of precious opal, backed by a layer of
dark-colored material, most commonly ironstone, dark or black common
opal (potch), onyx, or obsidian. The darker backing emphasizes the play
of color, and results in a more attractive display than a lighter potch.
An opal triplet is similar to a doublet, but has a third layer, a domed
cap of clear quartz or plastic on the top. The cap takes a high polish
and acts as a protective layer for the opal. The top layer also acts as a
magnifier, to emphasize the play of color of the opal beneath, which is
often of lower quality. Triplet opals therefore have a more artificial
appearance, and are not classed as precious opal. Jewelry applications
of precious opal can be somewhat limited by opal's sensitivity to heat
due primarily to its relatively high water content and predisposition to
Combined with modern techniques of polishing, doublet opal produces a
similar effect to black or boulder opal at a 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.
gemstone varieties that show a play of color, the other kinds of common
opal include the milk opal, milky bluish to greenish (which can
sometimes be of gemstone quality); resin opal, which is honey-yellow
with a resinous luster; wood opal, which is caused by the replacement of
the organic material in wood with opal; menilite, which is brown or
grey; hyalite, a colorless glass-clear opal sometimes called Muller's
glass; geyserite, also called siliceous sinter, deposited around hot
springs or geysers; and diatomite or diatomaceous earth, the
accumulations of diatom shells or tests.