Opal

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Opal
Opal-53714.jpg
A rich seam of iridescent opal encased in matrix
General
Category Mineraloid
Formula
(repeating unit)
Hydrated silica. SiO2·nH2O
Crystal system Amorphous [1]
Identification
ColorColorless, white, yellow, red, orange, green, brown, black, blue, pink
Crystal habit Irregular veins, in masses, in nodules
Cleavage None [1]
Fracture Conchoidal to uneven [1]
Mohs scale hardness5.5–6 [1]
Luster Subvitreous to waxy [1]
Streak White
Diaphaneity opaque, translucent, transparent
Specific gravity 2.15+0.08
−0.90
[1]
Density 2.09
Polish lusterVitreous to resinous [1]
Optical propertiesSingle refractive, often anomalous double refractive due to strain [1]
Refractive index 1.450+0.020
−0.080

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: 660 nm, 470 nm cutoff [1]
Diagnostic featuresdarkening upon heating
Solubility hot salt water, bases, methanol, humic acid, hydrofluoric acid
References [2] [3]
Common Rough Opal Common Rough Opal.jpg
Common Rough Opal

Opal 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 Australia. [4]

In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understood.

Silicon dioxide chemical compound

Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula SiO2, most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as synthetic product. Notable examples include fused quartz, fumed silica, silica gel, and aerogels. It is used in structural materials, microelectronics (as an electrical insulator), and as components in the food and pharmaceutical industries.

A mineraloid is a naturally-occurring mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals. For example, obsidian is an amorphous glass and not a crystal. Jet is derived from decaying wood under extreme pressure. Opal is another mineraloid because of its non-crystalline nature. Pearl, considered by some to be a mineral because of the presence of calcium carbonate crystals within its structure, would be better considered a mineraloid because the crystals are bonded by an organic material, and there is no definite proportion of the components.

Contents

There are two broad classes of opal: precious and common. Precious opal displays play-of-color (iridescence), common opal does not. [5] Play-of-color is defined as "a pseudochromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light." [6] The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Depending on the conditions in which it formed, opal may be transparent, translucent or opaque and the background color may be white, black or nearly any color of the visual spectrum. Black opal is considered to be the rarest, whereas white, gray and green are the most common.

Iridescence property in which fine colors, changeable with the angle of view or angle of illumination, are produced on a surface by the interference of light that is reflected from both the front and back of a thin film

Iridescence is the phenomenon of certain surfaces that appear to gradually change colour as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, butterfly wings and seashells, as well as certain minerals. It is often created by structural coloration.

Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit

Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit. It is defined as the bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wave. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1660.

Precious opal

Precious opal consists of spheres of silicon dioxide molecules arranged in regular, closely packed planes. (Idealized diagram) Opal molecular structure2.jpg
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 Coober Pedy Opal 2.jpg
Multicolor rough crystal opal from Coober Pedy, South Australia, expressing nearly every color of the visible spectrum
Precious opal replacing ichthyosaur backbone; display specimen, South Australian Museum Opalised Ichthyosaur backbone.jpg
Precious opal replacing ichthyosaur backbone; display specimen, South Australian Museum

Precious 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 [7] [8] that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal. [9] 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.

The nanometre or nanometer is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre. It can be written in scientific notation as 1×10−9 m, in engineering notation as 1 E−9 m, and as simply 1/1000000000 metres. When used as a prefix for something other than a unit of measure, nano refers to nanotechnology, or phenomena typically occurring on a scale of nanometres.

Crystal structure Ordered arrangement of atoms, ions, or molecules in a crystalline material

In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter.

Diffraction grating optical component which splits light into several beams

In optics, a diffraction grating is an optical component with a periodic structure that splits and diffracts light into several beams travelling in different directions. The emerging coloration is a form of structural coloration. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as the dispersive element. Because of this, gratings are commonly used in monochromators and spectrometers.

Visible light 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. [10] In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly used to describe this unique and beautiful phenomenon, which in gemology is termed play of color. In gemology, opalescence is applied to the hazy-milky-turbid sheen of common or potch opal which does not show a play of color. Opalescence is a form of adularescence.

Band gap energy range in a solid where no electron states can exist; energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors

In solid-state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. It is the energy required to promote a valence electron bound to an atom to become a conduction electron, which is free to move within the crystal lattice and serve as a charge carrier to conduct electric current. It is closely related to the HOMO/LUMO gap in chemistry. If the valence band is completely full and the conduction band is completely empty, then electrons cannot move in the solid; however, if some electrons transfer from the valence to the conduction band, then current can flow. Therefore, the band gap is a major factor determining the electrical conductivity of a solid. Substances with large band gaps are generally insulators, those with smaller band gaps are semiconductors, while conductors either have very small band gaps or none, because the valence and conduction bands overlap.

Photonic crystal a periodic optical nanostructure that affects the motion of photons in much the same way that ionic lattices affect electrons in solids

A photonic crystal is a periodic optical nanostructure that affects the motion of photons in much the same way that ionic lattices affect electrons in solids. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, in different forms, promise to be useful in a range of applications.

Vasily Astratov is a full professor of Physics and Optical Science at the University of North Carolina at Charlotte. He became known for launching synthetic opals as new self-assembled photonic crystals for visible light in 1995 in his former group at Ioffe Institute in Russia. This work has resulted in a quest for inverse opals with a complete three-dimensional photonic band gap.

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 scratching. [11] 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 Piece of mineral crystal used to make jewelry

A gemstone is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks and occasionally organic materials that are not minerals are also used for jewelry and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone.

A cabochon is a gemstone which has been shaped and polished as opposed to faceted. The resulting form is usually a convex (rounded) obverse with a flat reverse.

Quartz mineral composed of silicon and oxygen atoms in a continuous framework of SiO₄ silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO₂

Quartz is a mineral composed of silicon and oxygen atoms in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar.

Common opal

White and blue opal from Slovakia SLOVAKIAN OPAL 12.jpg
White and blue opal from Slovakia

Besides the 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; [12] 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 diatomaceous earth, the accumulations of diatom shells or tests. Common opal often displays a hazy-milky-turbid sheen from within the stone. In gemology, this optical effect is strictly defined as opalescence which is a form of adularescence.

Resin solid or highly viscous substance of plant or synthetic origin

In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on naturally-occurring resins.

Wood opal

Wood opal is a form of petrified wood which has developed an opalescent sheen or, more rarely, where the wood has been completely replaced by opal. Other names for this opalized sheen-like wood are opalized wood and opalized petrified wood. It is often used as a gemstone.

Menilite opal var.

Menilite is a greyish-brown form of the mineraloid opal. It is also known as liver opal or leberopal (German), due to its color. It is called menilite because it was first described from Ménilmontant (Paris), France, where it occurs as concretions within bituminous Early Oligocene Menilite Shales.

Other varieties of opal

Brightness of the fire in opal ranges on a scale of 1 to 5 (with 5 being the brightest) 10 7cts Brazilian Crystal Opal.jpg
Brightness of the fire in opal ranges on a scale of 1 to 5 (with 5 being the brightest)

Fire opal is a transparent to translucent opal, with warm body colors of yellow to orange to red. Although it does not usually show any play of color, 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 rhyolitic host material if it is hard enough to allow cutting and polishing. This type of Mexican opal is referred to as a Cantera opal. Also, a type of opal from Mexico, referred to as Mexican water opal, is a colorless opal which exhibits either a bluish or golden internal sheen. [14]

Girasol opal is a term sometimes mistakenly and improperly used to refer to fire opals, as well as a type of transparent to semitransparent type milky quartz from Madagascar which displays an asterism, or star effect, when cut properly. However, the true girasol opal [14] is a type of hyalite 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, too, 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 play of color. Blue opal also comes from Oregon in the Owyhee region, as well as from Nevada around the Virgin Valley.[ citation needed ]

Opal is also formed by diatoms. Diatoms are a form of algae which, when they die, often form layers at the bottoms of lakes, bays, or oceans. Their cell walls are made up of hydrated silicon dioxide which gives them structural coloration and therefore the appearance of tiny opals when viewed under a microscope. These cell walls or "tests" form the “grains” for the diatomaceous earth. This sedimentary rock is white, opaque, and chalky in texture. [15] Diatomite has multiple industrial uses such as filtering or adsorbing since it has a fine particle size and very porous nature, and gardening to increase water adsorption.

History

Opal was rare and very valuable in antiquity. In Europe it was a gem prized by royalty. [16] [17] Until the opening of vast deposits in Australia in the 19th century the only known source was Červenica beyond the Roman frontier in Slovakia. [18]

Sources

Polished opal from Yowah (Yowah Nut ), Queensland Opal from Yowah, Queensland, Australia 2.jpg
Polished opal from Yowah (Yowah Nut ), Queensland
Boulder opal, Carisbrooke Station near Winton, Queensland Boulder Opal.jpg
Boulder opal, Carisbrooke Station near Winton, Queensland

Australian opal has often been cited as accounting for 95–97% of the world's supply of precious opal, [20] [21] with the state of South Australia accounting for 80% of the world's supply. [22] Recent data suggests that the world supply of precious opal may have changed. In 2012, Ethiopian opal production was estimated to be 14,000 kg (31,000 lb) by the United States Geological Survey. [23] USGS data from the same period (2012), reveals that Australian opal production to be $41 million. [24] Because of the units of measurement, it is not possible to directly compare Australian and Ethiopian opal production, but these data and others suggest that the traditional percentages given for Australian opal production may be overstated. [25] Yet, the validity of data in the USGS report appears to conflict with that of Laurs et al.[ citation needed ] and Mesfin,[ citation needed ] who estimated the 2012 Ethiopian opal output (from Wegal Tena) to be only 750 kg (1,650 lb).

Australia

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 (3.4 kg; 7.5 lb) and is 11 inches (280 mm) long, with a height of 4 34 in (120 mm) and a width of 4 12 in (110 mm). [26] The Mintabie Opal Field located about 250 km (160 mi) north west of Coober Pedy has also produced large quantities of crystal opal and 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. [27] Its largest quantities are found around Jundah and Quilpie in South West Queensland. Australia also has opalized fossil remains, including dinosaur bones in New South Wales [28] and South Australia, [29] and marine creatures in South Australia. [30]

Ethiopia

Although it has been reported that Northern African opal was used to make tools as early as 4000 BC, the first published report of gem opal from Ethiopia appeared in 1994, with the discovery of precious opal in the Menz Gishe District, North Shewa Province. [31] The opal, found mostly in the form of nodules, was of volcanic origin and was found predominantly within weathered layers of rhyolite. [32] This Shewa Province opal was mostly dark brown in color and had a tendency to crack. These qualities made it unpopular in the gem trade. In 2008, a new opal deposit was found approximately 180 km north of Shewa Province, near the town of Wegel Tena, in Ethiopia's Wollo Province. The Wollo Province opal was different from the previous Ethiopian opal finds in that it more closely resembled the sedimentary opals of Australia and Brazil, with a light background and often vivid play-of-color. [33] Wollo Province opal, more commonly referred to as "Welo" or "Wello" opal, has become the dominant Ethiopian opal in the gem trade. [34]

Gem grade Ethiopian Welo precious opal pendant Jupiter 20 Opal and Diamond Pendant.jpg
Gem grade Ethiopian Welo precious opal pendant

Virgin Valley, Nevada

Multicolored rough opal specimen from Virgin Valley, Nevada, US Nev opal09.jpg
Multicolored rough opal specimen from Virgin Valley, Nevada, US

The Virgin Valley [35] 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 in situ within a subsurface horizon or zone of bentonite, which is considered a "lode" deposit. Opals which have weathered out of the in situ 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, [36] Royal Peacock, [37] Bonanza, [38] Opal Queen, [39] and WRT Stonetree/Black Beauty [40] mines. The largest unpolished black opal in the Smithsonian Institution, known as the "Roebling opal", [41] came out of the tunneled portion of the Rainbow Ridge Mine in 1917, and weighs 2,585 carats (517.0 g; 18.24 oz). The largest polished black opal in the Smithsonian Institution comes from the Royal Peacock opal mine in the Virgin Valley, weighing 160 carats (32 g; 1.1 oz), known as the "Black Peacock". [42]

Mexico

Opal occurs in significant quantity and variety in central Mexico, where mining and production center on the state of Querétaro. In this region the opal deposits are located mainly in the mountain ranges of three municipalities: Colón, Tequisquiapan and Ezequiel Montes. During the 1960s through to the mid-1970s the Querétaro mines were heavily mined. Today's opal miners report that it was much easier to find quality opals with a lot of fire and play of color back then, whereas today the gem quality opals are very hard to come by and command hundreds of US dollars or more.

The oldest mine in Querétaro is Santa Maria del Iris. This mine was opened around 1870 and has been reopened at least 28 times since. At the moment there are about 100 mines in the regions around Querétaro, but most of them are now closed. The best quality of opals came from the mine Santa Maria del Iris, followed by La Hacienda la Esperanza, Fuentezuelas, La Carbonera and La Trinidad. Important deposits in the state of Jalisco were not discovered until the late 1950s.

In 1957, Alfonso Ramirez (of Querétaro) accidentally discovered the first opal mine in Jalisco - La Unica, located on the outer area of the volcano of Tequila, near the Huitzicilapan farm in Magdalena. By 1960 there were around 500 known opal mines in this region alone. Other regions of the country that also produce opals (of a lesser quality) are Guerrero, which produces an opaque opal similar to the opals from Australia (some of these opals are carefully treated with heat to improve their colors so high-quality opals from this area may be suspect). There are also some small opal mines in Morelos, Durango, Chihuahua, Baja California, Guanajuato, Puebla, Michoacán, and Estado de México.

Other locations

Another source of white base opal or creamy opal in the United States is Spencer, Idaho. [43] 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, Canada, Slovakia, Hungary, Turkey, Indonesia, Brazil (in Pedro II, Piauí [44] ), Honduras (more precisely in Erandique), Guatemala and Nicaragua.

In late 2008, NASA announced it had discovered opal deposits on Mars. [45]

Synthetic opal

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. [9] 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 ultraviolet light. Synthetics are also generally lower in density and are often highly porous.

Two notable producers of synthetic opal are 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 (such as 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. [46]

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, no unit cell 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. [47] 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 SiOH, generally form a lesser proportion of the total and can reside near the surface or in defects inside the opal.

The crystal structure of crystalline a-cristobalite. Locally, the structures of some opals, opal-C, are similar to this. Alphacrist.png
The crystal structure of crystalline α-cristobalite. Locally, the structures of some opals, opal-C, are similar to this.

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

Lussatite (opal-CT) Lussatite-France.jpg
Lussatite (opal-CT)

Opal-CT has been interpreted as consisting of clusters of stacked cristobalite and tridymite over very short length scales. The spheres of opal in opal-CT are themselves made up of tiny nanocrystalline blades of cristobalite and tridymite. Opal-CT has occasionally been further subdivided in the literature. Water content may be as high as 10 wt%. [48] Opal-C, also called lussatine or lussatite, is interpreted as consisting of localized order of α-cristobalite with a lot of stacking disorder. Typical water content is about 1.5 wt%.

Noncrystalline opal

Two broad categories of noncrystalline 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.

Noncrystalline 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. [49]

Etymology

The word 'opal' is adapted from the Latin 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. [50]

References to the gem are made by Pliny the Elder. It is suggested to have been adapted 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 Ancient 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 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. [50]

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. [51] It was also said to grant invisibility if wrapped in a fresh bay leaf and held in the hand. [51] [52] Following the publication of Sir Walter Scott's Anne of Geierstein in 1829, 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. [51] 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 20 years or so. [53]

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, as the opal was believed to embody the evil eye. [51]

Opal is considered the birthstone for people born in October. [54]

Examples

See also

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Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

Chalcedony Microcrystalline varieties of quartz, may contain moganite as well

Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO2 (silicon dioxide).

Lustre or luster is the way light interacts with the surface of a crystal, rock, or mineral. The word traces its origins back to the Latin lux, meaning "light", and generally implies radiance, gloss, or brilliance.

Jasper Chalcedony variety colored by iron oxide

Jasper, an aggregate of microgranular quartz and/or chalcedony and other mineral phases, is an opaque, impure variety of silica, usually red, yellow, brown or green in color; and rarely blue. The common red color is due to iron(III) inclusions. The mineral aggregate breaks with a smooth surface and is used for ornamentation or as a gemstone. It can be highly polished and is used for items such as vases, seals, and snuff boxes. The specific gravity of jasper is typically 2.5 to 2.9. A green variety with red spots, known as heliotrope (bloodstone), is one of the traditional birthstones for March. Jaspilite is a banded iron formation rock that often has distinctive bands of jasper.

Coober Pedy Town in South Australia

Coober Pedy is a town in northern South Australia, 846 km (526 mi) north of Adelaide on the Stuart Highway. In the 2016 Census, there were 1,762 people in Coober Pedy. Of these, 962 were male and 801 were female. There were 302 Aboriginal and/or Torres Strait Islander people that made up 17.1% of the population. The town is sometimes referred to as the "opal capital of the world" because of the quantity of precious opals that are mined there. Coober Pedy is renowned for its below-ground residences, called "dugouts", which are built in this fashion due to the scorching daytime heat. The name "Coober Pedy" comes from the local Aboriginal term kupa-piti, which means "boys' waterhole".

Onyx Banded variety of the mineral chalcedony

Onyx primarily refers to the parallel banded variety of the silicate mineral chalcedony. Agate and onyx are both varieties of layered chalcedony that differ only in the form of the bands: agate has curved bands and onyx has parallel bands. The colors of its bands range from white to almost every color. Commonly, specimens of onyx contain bands of black and/or white. Onyx, as a descriptive term, has also been applied to parallel banded varieties of alabaster, marble, obsidian and opal, and misleadingly to materials with contorted banding, such as "Cave Onyx" and "Mexican Onyx".

Tanzanite Blue to purple variety of the mineral zoisite

Tanzanite is the blue and violet variety of the mineral zoisite, caused by small amounts of vanadium, belonging to the epidote group. Tanzanite is only found in Tanzania, in a very small mining area near the Merelani Hills.

Cristobalite silica mineral

Cristobalite is a mineral polymorph of silica that is formed at very high-temperatures. It is used in dentistry as a component of alginate impression materials as well as for making models of teeth

Variscite phosphate mineral

Variscite is a hydrated aluminium phosphate mineral (AlPO4·2H2O). It is a relatively rare phosphate mineral. It is sometimes confused with turquoise; however, variscite is usually greener in color. The green color results from the presence of small amounts of chromium.

Chrysocolla phyllosilicate mineral

Chrysocolla is a hydrated copper phyllosilicate mineral with formula: Cu2−xAlx(H2−xSi2O5)(OH)4·nH2O (x<1) or (Cu,Al)2H2Si2O5(OH)4·nH2O. The structure of the mineral has been questioned, as spectrographic studies suggest material identified as chrysocolla may be a mixture of the copper hydroxide spertiniite and chalcedony.

Tridymite silica mineral

Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is SiO2. Tridymite was first described in 1868 and the type location is in Hidalgo, Mexico. The name is from the Greek tridymos for triplet as tridymite commonly occurs as twinned crystal trillings (compound crystals comprising three twinned crystal components).

Opalescence

Opalescence refers to the optical phenomena displayed by the mineraloid gemstone opal. However, there are three notable types of opal, each with different optical effects, so the intended meaning varies depending on context. The optical effects seen in various types of opal are a result of refraction or reflection (common) due to the layering, spacing, and size of the myriad microscopic silicon dioxide spheres and included water in its physical structure. When the size and spacing of the silica spheres are relatively small, refracted blue-green colors are prevalent; when relatively larger, refracted yellow-orange-red colors are seen; and when larger yet, reflection yields a milky-hazy sheen.

Ammolite

Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral contained in nacre, with a microstructure inherited from the shell. It is one of few biogenic gemstones; others include amber and pearl.1 In 1981, ammolite was given official gemstone status by the World Jewellery Confederation (CIBJO), the same year commercial mining of ammolite began. It was designated the official gemstone of the City of Lethbridge, Alberta in 2007.

Adularescence

Adularescence is an optical phenomenon, similar to labradorescence and aventurescence, that is produced in gemstones such as moonstones.

The Olympic Australis opal is the largest and most valuable opal yet found, valued at A$2,500,000. It was found in 1956 at the 'Eight Mile' opal field in the outback opal gemstone mining town of Coober Pedy in South Australia.

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Further reading