Opal is a hydrated amorphous form of silica; 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 the other 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 Australia, which produces 97% of the world's supply.
This includes the production of the state of South Australia, which accounts for approximately
80% of the world's supply. 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 reds against black are
the most rare, whereas white and greens are the most common. It varies in optical density
from opaque to semi-transparent. Common opal, called "potch" by miners, does
not show the display of color exhibited in precious opal.
Precious opal
Precious opal shows a variable interplay of internal colors and even though it is a mineraloid,
it has an internal structure. At micro scales precious opal is composed of silica spheres
some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. These ordered
silica spheres produce the internal colors by causing the interference and diffraction
of light passing through the microstructure of the opal. It is the regularity of the sizes
and 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. 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, 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. Combined with modern techniques 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. 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, 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. Other varieties of opal
Fire opal is a transparent to translucent opal, with warm body colors of yellow, orange,
orange-yellow or red. It does not usually show any play of color, although occasionally
a stone will exhibit bright green flashes. The most famous source of fire opals is the
state of Querétaro in Mexico; these opals are commonly called Mexican fire opals. Fire
opals that do not show play of color are sometimes referred to as jelly opals. Mexican opals
are sometimes cut in their ryholitic host material if it is hard enough to allow cutting
and polishing. This type of Mexican opal is referred to as a Cantera Opal. There is also
a type of opal from Mexico referred to as Mexican Water Opal, which is a colorless opal
which exhibits either a bluish or golden internal sheen.
Girasol opal is a term sometimes mistakenly and improperly used to refer to fire opals
as well as a type of transparent to semi-transparent type milky quartz from Madagascar which displays
an asterism, or star effect, when cut properly. However, there is a true girasol opal that
is a type of halite opal, that exhibits a bluish glow or sheen that follows the light
source around. It is not a play of color as seen in precious opal but rather an effect
from microscopic inclusions. It is also sometimes referred to as water opal as well when it
is from Mexico. The two most notable locations of this type of opal are Oregon and Mexico.
Peruvian 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. Blue opal
also comes from Oregon in the Owhyee region as well as from Nevada around Virgin Valley.
Sources of opal
Australia produces around 97% of the world's opal. 90% is called 'light opal' or white
and crystal opal. White makes up 60% of the opal productions but cannot be found in all
of the opal fields. Crystal opal or pure hydrated silica makes up 30% of the opal produced,
8% is black and only 2% is boulder opal. The town of Coober Pedy in South Australia
is a major source of opal. The world's largest and most valuable gem opal "Olympic Australis"
was found in August 1956 at the "Eight Mile" opal field in Coober Pedy. It weighs 17,000
carats and is 11 inches long, with a height of 4 3⁄4 inches and a width of 4 1⁄2
inches.
The Mintabie Opal Field located approximately 250 km north west of Coober Pedy has also
produced large quantities of crystal opal and also the rarer black opal. Over the years
it has been sold overseas incorrectly as Coober Pedy Opal. The black opal is said to be some
of the best examples found in Australia. 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. 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. Its largest quantities are found around Jundah
and Quilpie in South West Queensland. Australia also has opalised fossil remains, including
dinosaur bones in New South Wales, and marine creatures in South Australia. The rarest type
of Australian opal is "pipe" opal, closely related to boulder opal, which forms in sandstone
with some iron-oxide content, usually as fossilized tree roots.
The Virgin Valley opal fields of Humboldt County in northern Nevada produce a wide variety
of precious black, crystal, white, fire, and lemon opal. The black fire opal is the official
gemstone of Nevada. Most of the precious opal is partial wood replacement. The precious
opal is hosted and found within a subsurface horizon or zone of bentonite in-place which
is considered a "lode" deposit. Opals which have weathered out of the in-place deposits
are alluvial and considered placer deposits. Miocene age opalised teeth, bones, fish, and
a snake head have been found. Some of the opal has high water content and may desiccate
and crack when dried. The largest producing mines of Virgin Valley have been the famous
Rainbow Ridge, Royal Peacock, Bonanza, Opal Queen, and WRT Stonetree/Black Beauty Mines.
The largest unpolished Black Opal in the Smithsonian Institution, known as the "Roebling Opal,"
came out of the tunneled portion of the Rainbow Ridge Mine in 1917, and weighs 2,585 carats.
The largest polished black opal in the Smithsonian Institution comes from the Royal Peacock opal
mine in the Virgin Valley, weighing 160 carats, known as the "Black Peacock."
Another source of white base opal or creamy opal in the United States is Spencer, Idaho.
A high percentage of the opal found there occurs in thin layers.
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. Furthermore, synthetic opals do not fluoresce
under UV light. Synthetics are also generally lower in density and are often highly porous.
Two notable producers of synthetic opal are the companies Kyocera and Inamori of Japan.
Most so-called synthetics, however, are more correctly termed "imitation opal", as they
contain substances not found in natural opal. The imitation opals seen in vintage jewelry
are often foiled glass, glass-based "Slocum stone", or later plastic materials.
Other research in macroporous structures have yielded highly ordered materials that have
similar optical properties to opals and have been used in cosmetics.
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
order and 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 SiO2·nH2O 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, also called Lussatine, is interpreted as consisting of localized
order of -cristobalite with a lot of stacking disorder. Typical water content is about 1.5wt%.
Non-crystalline opal Two broad categories of non-crystalline opals,
sometimes just referred to as "opal-A", have been proposed. The first of these is opal-AG
consisting of 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. The second "opal-A" is opal-AN or water-containing
amorphous silica-glass. Hyalite is another name for this.
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.
Naming The word opal is adapted from the Roman term
opalus, but the origin of this word is a matter of debate. However, most modern references
suggest it is adapted from the Sanskrit word úpala.
References to the gem are made by Pliny the Elder. It is suggested it was adapted it from
Ops, the wife of Saturn and goddess of fertility. The portion of Saturnalia devoted to Ops was
"Opalia", similar to opalus. Another common claim that the term is adapted
from the Greek word, opallios. This word has two meanings, one is related to "seeing" and
forms the basis of the English words like "opaque", the other is "other" as in "alias"
and "alter". It is claimed that opalus combined these uses, meaning "to see a change in color".
However, historians have noted that the first appearances of opallios do not occur until
after the Romans had taken over the Greek states in 180 BC, and they had previously
used the term paederos. However, the argument for the Sanskrit origin
is strong. The term first appears in Roman references around 250 BC, at a time when the
opal was valued above all other gems. The opals were supplied by traders from the Bosporus,
who claimed the gems were being supplied from India. Before this the stone was referred
to by a variety of names, but these fell from use after 250 BC.
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. It was
also said to confer the power of invisibility if wrapped in a fresh bay leaf and held in
the hand. Following the publication of Sir Walter Scott's Anne of Geierstein in 1829,
however, opal acquired a less auspicious reputation. In Scott's novel, the Baroness of Arnheim
wears an opal talisman with supernatural powers. When a drop of holy water falls on the talisman,
the opal turns into a colorless stone and the Baroness dies soon thereafter. Due to
the popularity of Scott's novel, people began to associate opals with bad luck and death.
Within a year of the publishing of Scott's novel in April 1829, the sale of opals in
Europe dropped by 50%, and remained low for the next twenty years or so.
Even as recently as the beginning of the 20th century, it was believed that when a Russian
saw an opal among other goods offered for sale, he or she should not buy anything more
since the opal was believed to embody the evil eye.
Opal is considered the birthstone for people born in October or under the sign of Scorpio
and Libra. Famous opals
The Olympic Australis, the world's largest and most valuable gem opal
The Andamooka Opal, presented to Queen Elizabeth II, also known as the Queen's Opal
The Addyman Plesiosaur from Andamooka, "the finest known opalised skeleton on Earth"
The Burning of Troy, the now-lost opal presented to Joséphine de Beauharnais by Napoleon I
of France and the first named opal The Flame Queen Opal
The Halley's Comet Opal, the world's largest uncut black opal
Although the clock faces above the information stand in Grand Central Terminal Manhattan,
New York, are often said to be opal, they are in fact opalescent glass
The Roebling Opal, Smithsonian Institution The Galaxy Opal, listed as the "World's Largest
Polished Opal" in the 1992 Guinness Book of Records
See also Opalite
Optical phenomena References
External links Farlang opal Hist. References Localities,
anecdotes by Theophrastus, Isaac Newton, Georg Agricola etc.
ICA's Opal Page: International Colored Stone Association
South Australian Museum > The Opal Fossils of South Australia Accessed 2 December 2011.