Opal
| Opal |
|
| General |
| Category |
Mineraloid |
Formula
(repeating unit) |
Hydrated silica. SiO2·nH2O |
| Identification |
| Color |
Colorless, white, yellow, red, orange, green, brown, black, blue |
| Crystal habit |
Irregular veins, in masses, in nodules |
| Crystal system |
Amorphous[1] |
| Cleavage |
None[1] |
| Fracture |
Conchoidal to uneven[1] |
| Mohs scale hardness |
5.5–6[1] |
| Luster |
Subvitreous to waxy[1] |
| Streak |
White |
| Diaphaneity |
opaque, translucent, transparent |
| Specific gravity |
2.15 (+.08, -.90)[1] |
| Density |
2.09 |
| Polish luster |
Vitreous to resinous[1] |
| Optical properties |
Single refractive, often anomalous double refractive due to strain[1] |
| Refractive index |
1.450 (+.020, -.080) Mexican opal may read as low as 1.37, but typically reads 1.42–1.43[1] |
| Birefringence |
none[1] |
| Pleochroism |
None[1] |
| Ultraviolet fluorescence |
black or white body color: inert to white to moderate light blue, green, or yellow in long and short wave. May also phosphoresce; common opal: inert to strong green or yellowish green in long and short wave, may phosphoresce; fire opal: inert to moderate greenish brown in long and short wave, may phosphoresce.[1] |
| Absorption spectra |
green stones: 660nm, 470nm cutoff[1] |
| Diagnostic features |
darkening upon heating |
| Solubility |
hot saltwater, bases, methanol, humic acid, hydrofluoric acid |
| References |
[2][3] |
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.
[4] This includes the production of the state of
South Australia, which accounts for approximately 80% of the world's supply.
[5]
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.
[6]
Precious opal
Precious opal consists of spheres of silicon dioxide molecules arranged in regular, closely packed planes. (Idealized diagram)
Multicolor Rough Crystal Opal from Coober Pedy, South Australia, expressing nearly every color of the visible spectrum.
Brightness of the fire in Opal ranges on a scale of 1 to 5 (5 being the brightest)
[7]
Australian Opal Doublet, a slice of precious opal with a backing of ironstone.
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.
[8]
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 (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.
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
A piece of milky raw opal from Andamooka South Australia
A cabochon cut from a piece of opalized wood
An opal "triplet" from Andamooka South Australia showing blue and green fire
A rock showing striations of opal throughout
A close-up view of striations within 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, which is honey-yellow with a resinous luster;
wood opal, which is caused by the replacement of the organic material in
wood with opal;
[9] 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.
[10]
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
[10]
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.
[citation needed]
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. Blue
opal also comes from Oregon in the Owhyee region as well as from Nevada
around Virgin Valley.
[citation needed]
Sources of opal
Polished opal from Yowah (Yowah Nut
[11]), Queensland, Australia
Multi-Colored solid black opal cabochon from Lightning Ridge, NSW
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.
[citation needed]
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
(3450 grams) and is 11 inches (280 mm) long, with a height of
4 3⁄4 inches (120 mm) and a width of
4 1⁄2 inches (110 mm).
[citation needed]
Boulder Opal, Carisbrooke Station near Winton, Queensland
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 (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.
[12] Its largest quantities are found around
Jundah and
Quilpie (known as the "home of the Boulder Opal"
[13]) in
South West Queensland.
Australia also has opalised fossil remains, including dinosaur bones in
New South Wales, and marine creatures in South Australia.
[14] 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.
[citation needed]
Multi-colored rough opal specimen from Virgin Valley, Nevada, US
The Virgin Valley
[15] 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,
[16] Royal Peacock,
[17] Bonanza,
[18] Opal Queen,
[19] and WRT Stonetree/Black Beauty
[20] Mines. The largest unpolished Black Opal in the Smithsonian Institution, known as the "Roebling Opal,"
[21]
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."
[22]
Another source of
white base opal or
creamy opal in the United States is
Spencer, Idaho.
[citation needed] 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 (in
Pedro II, Piauí[23]), Honduras, Guatemala, Nicaragua and Ethiopia.
In late 2008,
NASA announced that it had discovered opal deposits on
Mars.
[24]
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.
[8]
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
(e.g., plastic stabilizers). 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.
[25]
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 (
amorphous opal) and those that show signs of the beginning of crystalline order, commonly termed
cryptocrystalline or microcrystalline opal.
[26] Dehydration experiments and
infrared spectroscopy have shown that most of the H
2O in the formula of SiO
2·nH
2O 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 SiO
4. 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.
The crystal structure of crystalline α-cristobalite. Locally, the structures of some opals, opal-C, are similar to this.
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.
[27]
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.[28]
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.
[28]
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.
[29] It was also said to confer the power of invisibility if wrapped in a fresh
bay leaf and held in the hand.
[29][30] 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.
[29]
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.
[31]
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.
[29]
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[citation needed]
- 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"[32]
- The Burning of Troy, the now-lost opal presented to Joséphine de Beauharnais by Napoleon I of France and the first named opal[33]
- 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[34]
- The Galaxy Opal, listed as the "World's Largest Polished Opal" in the 1992 Guinness Book of Records[35]
