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[2]
Opal is ahydratedamorphous form ofsilica (SiO2·nH2O); itswater content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as amineraloid, unlikecrystalline forms of silica, which are consideredminerals. It is deposited at a relatively low temperature and may occur in thefissures of almost any kind ofrock, being most commonly found withlimonite,sandstone,rhyolite,marl, andbasalt.
The nameopal is believed to be derived from theSanskrit wordupala (उपल), which means 'jewel', and later the Greek derivativeopállios (ὀπάλλιος).[5]
There are two broad classes of opal: precious and common. Precious opal displays play-of-color (iridescence); common opal does not.[6] Play-of-color is defined as "a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light."[7] The internal structure of precious opal causes it todiffract 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 the rarest, while white, gray, and green opals are the most common.
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 150–300 nm (5.9×10−6–1.18×10−5 in) in diameter in a hexagonal or cubicclose-packedlattice. It was shown by J. V. Sanders in the mid-1960s[8][9] that these ordered silica spheres produce the internal colors by causing theinterference anddiffraction of light passing through the microstructure of the opal.[10] The regularity of the sizes and the packing of these spheres is a prime determinant of the quality of precious opal. Where the distance between the regularly packed planes of spheres is around half the wavelength of a component ofvisible light, the light of that wavelength may be subject to diffraction from thegrating 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 byBragg's law of diffraction.
Visible light cannot pass through large thicknesses of the opal. This is the basis of the opticalband gap in aphotonic crystal.[11] In addition, microfractures may be filled with secondary silica and form thinlamellae inside the opal during its formation. The termopalescence is commonly used to describe this unique and beautiful phenomenon, which in gemology is termedplay 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.[clarification needed] Opalescence is a form ofadularescence.
For use in jewellery, most opal is cut and polished to form acabochon, a convex shape that maximises the viewing angles through which an opal's play of colour can be observed. "Natural" opal refers to polished stones consisting wholly of precious opal. Opals too thin to produce a "natural" opal may be combined with other materials to form "composite" gems. An opaldoublet 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 clearquartz 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 an inferior specimen or an extremely thin section of precious opal. Triplet opals tend to have a more artificial appearance and are not classed as precious gemstones, but rather "composite" gemstones. 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.[12] Combined with modern techniques of polishing, a doublet opal can produce a similar effect to Natural 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.
Common rough opalWhite and blue opal from Slovakia
Besides thegemstone 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;[13]menilite, which is brown or grey;hyalite, a colorless glass-clear opal sometimes called Muller's glass;geyserite, also calledsiliceous sinter, deposited aroundhot springs orgeysers; anddiatomaceous earth, the accumulations ofdiatom shells or tests. Common opal often displays a hazy-milky-turbid sheen from within the stone. Ingemology, this optical effect is strictly defined as opalescence which is a form of adularescence.
"Girasol" redirects here. For the American sunflower, seeJerusalem artichoke.
Brazilian Precious Opal with patches of brilliant color throughout. The brightness of the fire in opal ranges on a scale of 1 to 5 (with 5 being the brightest)[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[15] is a type ofhyalite 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 areOregon and Mexico.[16]
A 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 a play of color. Blue opal also comes fromOregon and Idaho in theOwyhee region, as well as fromNevada around theVirgin Valley.[17]
Opal is also formed bydiatoms. Diatoms are a form of algae that, 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 themstructural coloration and therefore the appearance of tiny opals when viewed under a microscope. These cell walls or "tests" form the "grains" for thediatomaceous earth. This sedimentary rock is white, opaque, and chalky in texture.[18] 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 absorption.
Opal was rare and very valuable in antiquity. In Europe, it was a gem prized by royalty.[19][20] Until the opening of vast deposits in Australia in the 19th century the only known source wasČervenica beyond the Roman frontier inSlovakia.[21] Opal is the nationalgemstone of Australia.[22]
The primary sources of opal are Australia and Ethiopia, but because of inconsistent and widely varying accountings of their respective levels of extraction, it is difficult to accurately state what proportion of the global supply of opal comes from either country.Australian opal has been cited as accounting for 95–97% of the world's supply of precious opal,[24][25] with the state ofSouth Australia accounting for 80% of the world's supply.[26] In 2012, Ethiopian opal production was estimated to be 14,000 kg (31,000 lb) by the United States Geological Survey.[27]USGS data from the same period (2012), reveals Australian opal production to be $41 million.[28] 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.[29] 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 (fromWegeltena) to be only 750 kg (1,650 lb).
The town ofCoober Pedy inSouth 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 of4+3⁄4 in (120 mm) and a width of4+1⁄2 in (110 mm).[30]
TheMintabie Opal Field in South Australia located about 250 km (160 mi) northwest 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 inNew 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), collected from theGriman Creek Formation.[31] Boulder opal consists ofconcretions and fracture fillings in a dark siliceousironstone matrix. It is found sporadically in western Queensland, from Kynuna in the north, toYowah andKoroit in the south.[32] Its largest quantities are found aroundJundah andQuilpie inSouth West Queensland. Australia also has opalizedfossil remains, including dinosaur bones in New South Wales[33] and South Australia,[34] and marine creatures in South Australia.[35]
The first published report of gem opal fromEthiopia appeared in 1994, with the discovery of precious opal in the Menz Gishe District, North Shewa Province.[36] The opal, found mostly in the form of nodules, was of volcanic origin and was found predominantly within weathered layers ofrhyolite.[37] 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 ofWegeltena, in Ethiopia'sWollo 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.[38] Wollo Province opal, more commonly referred to as "Welo" or "Wello" opal, has become the dominant Ethiopian opal in the gem trade.[39]
Multicolored rough opal specimen from Virgin Valley, Nevada, US
The Virgin Valley[40] opal fields ofHumboldt County in northernNevada 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 foundin situ within a subsurface horizon or zone ofbentonite, which is considered a "lode" deposit. Opals which have weathered out of thein situ deposits are alluvial and consideredplacer 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,[41] Royal Peacock,[42] Bonanza,[43] Opal Queen,[44] and WRT Stonetree/Black Beauty[45] mines. The largest unpolished black opal in the Smithsonian Institution, known as the "Roebling opal",[46] 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 theSmithsonian 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".[47]
Fire opal is a transparent to translucent opal with warm body colors of yellow to orange to red. Although fire opals don't usually show any play of color, they occasionally exhibit bright green flashes. The most famous source of fire opals is the state ofQuerétaro in Mexico; these opals are commonly called Mexican fire opals.[48] Fire opals that do not show a 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. Another type of opal from Mexico, referred to as Mexican water opal, is a colorless opal that exhibits either a bluish or golden internal sheen.[15]
Precious Mexican Fire Opal has bright green and yellow patches (play of color) with an orange-red background typical of Fire Opal.
Opal occurs in significant quantity and variety in central Mexico, wheremining and production first originated in the state ofQuerétaro. In this region the opal deposits are located mainly in the mountain ranges of three municipalities:Colón,Tequisquiapan, andEzequiel 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 orange-red background color is characteristic of all "fire opals," including "Mexican fire opal".
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 ofJalisco 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 inMagdalena. By 1960 there were around 500 known opal mines in this region alone. Other regions of the country that also produce opals (of lesser quality) areGuerrero, 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 inMorelos,Durango,Chihuahua,Baja California,Guanajuato,Puebla,Michoacán, andEstado de México.
Another source of white base opal or creamy opal in the United States isSpencer, Idaho.[49] 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 (inPedro II, Piauí[50]), Honduras (more precisely inErandique), Guatemala, and Nicaragua.
In late 2008,NASA announced the discovery of opal deposits onMars.[51]
Wood opal, also known asxylopal,[52][53] is a form of opal,[54] as well as a type ofpetrified 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 areopalized wood andopalized petrified wood. It is often used as agemstone.[55]
Artificial opal (made by drying a monodisperse sub-micrometer polystyrene spheresol) viewed by darkfield optical microscopy
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.[10] 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 notfluoresce underultraviolet light.[citation needed] Synthetics are also generally lower in density and are often highly porous.
Opals which have been created in a laboratory are often termed "lab-created opals", which, while classifiable as man-made and synthetic, are very different from their resin-based counterparts which are also considered man-made and synthetic. The term "synthetic" implies that a stone has been created to be chemically and structurally indistinguishable from a genuine one, and genuine opal contains no resins or polymers. The finest modern lab-created opals do not exhibit the lizard skin or columnar patterning of earlier lab-created varieties, and their patterns are non-directional. They can still be distinguished from genuine opals, however, by their lack ofinclusions and the absence of any surrounding non-opalmatrix. While many genuine opals are cut and polished without a matrix, the presence of irregularities in their play-of-color continues to mark them as distinct from even the best lab-created synthetics.
Other research in macroporous structures have yielded highly ordered materials that have similar optical properties to opals and have been used in cosmetics.[56] Synthetic opals are also deeply investigated inphotonics for sensing and light management purposes.[57][58]
The crystal structure of crystalline α-cristobalite. Locally, the structures of some opals, opal-C, are similar to this.Lussatite (opal-CT)β-Tridymite, a hexagonal (P63/mmc) polymorph of silica
The lattice of spheres of opal that cause interference with light is several hundred times larger than the fundamental structure of crystalline silica. As amineraloid, nounit 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 termedcryptocrystalline or microcrystalline opal.[59] Dehydration experiments andinfrared 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, andsilanols, 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 structure of low-pressurepolymorphs of anhydroussilica consists of frameworks of fully corner bonded tetrahedra of SiO4. The higher temperature polymorphs of silicacristobalite andtridymite 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 ashexagonal andcubic close-packed layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.
Microcrystalline opal orOpal-CT has been interpreted as consisting of clusters of stackedcristobalite andtridymite over very short length scales. The spheres of opal in microcrystalline opal are themselves made up of tiny nanocrystalline blades of cristobalite and tridymite. Microcrystalline opal has occasionally been further subdivided in the literature. Water content may be as high as 10 wt%.[60] Opal-CT, also calledlussatine orlussatite, is interpreted as consisting of localized order of α-cristobalite with a lot of stacking disorder. Typical water content is about 1.5 wt%.
Two broad categories of noncrystalline opals, sometimes just referred to as "opal-A" ("A" stands for "amorphous"),[61] 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 ofdiagenesis, due to the increasing overburden pressure insedimentary rocks, as some of the stacking disorder is removed.[62]
Schematic representation of the hydrated opal surface.
The surface of opal in contact with water is covered bysiloxane bonds (≡Si–O–Si≡) andsilanol groups (≡Si–OH). This makes the opal surface veryhydrophilic and capable of forming numeroushydrogen bonds.
The word 'opal' is adapted from theLatin termopalus. The origin of this word in turn is a matter of debate, but most modern references suggest it is adapted from theSanskrit wordúpala meaning ‘precious stone’.[5]
As references to the gem are made byPliny the Elder, one theory attributed the name's origin to Roman mythology: to have been adapted fromOps, the wife ofSaturn, and goddess of fertility. (The portion ofSaturnalia devoted to Ops was "Opalia", similar toopalus.)
Another common claim was that the term was adapted from theAncient 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 thatopalus combined these uses, meaning "to see a change in color". However, historians have noted the first appearances ofopallios do not occur until after the Romans had taken over the Greek states in 180 BC and they had previously used the termpaederos.[5]
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 theBosporus, 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.
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.[63] It was also said to grant invisibility.[63] As a result, the opal was seen as the patron gemstone forthieves during themedieval period.[64] Following the publication of SirWalter Scott'sAnne of Geierstein in 1829, opal acquired a less auspicious reputation. In Scott's novel, the Baroness of Arnheim wears an opaltalisman with supernatural powers. When a drop ofholy 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.[63] 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.[65]
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 theevil eye.[63]
Opal is considered thebirthstone for people born in October.[66]
Although the clock faces above the information stand inGrand Central Terminal in New York City are often said to be opal, they are in factopalescent glass
^Astratov, V. N.; Bogomolov, V. N.; Kaplyanskii, A. A.; Prokofiev, A. V.; Samoilovich, L. A.; Samoilovich, S. M.; Vlasov, Yu. A. (1995). "Optical spectroscopy of opal matrices with CdS embedded in its pores: Quantum confinement and photonic band gap effects".Il Nuovo Cimento D.17 (11–12):1349–1354.Bibcode:1995NCimD..17.1349A.doi:10.1007/bf02457208.S2CID121167426.
^Dr. Joel Arem; Donald Clark, CSM IMG (23 June 2015)."Opal Value, Price, and Jewelry".Gemsociety.org.Archived from the original on 23 November 2016. Retrieved22 November 2016.
^Gribble, C. D. (1988). "Tektosilicates (framework silicates)".Rutley's Elements of Mineralogy (27th ed.). London: Unwin Hyman. p. 431.ISBN978-0-04-549011-0.
^Downing, Paul B. (1992).Opal Identification and Value. pp. 55–61.
^"The central stone on the thirteenth-century crown of the Holy Roman Emperor was an opal said to be the color of pure white snow, sparkling with splashes of bright red wine: it was called "the Orphanus", perhaps because there was no other stone like it.24 And on New Year's Day 1584,25 Queen Elizabeth I was delighted to receive an opal parure—a full set of matching jewelry—from one of her favorite courtiers, Sir Christopher Hatton." Finlay, Victoria. Jewels: A Secret History (Kindle Locations 2145–2148). Random House Publishing Group. Kindle Edition.
^"All of these early stones were almost certainly from the ancient mines of Slovakia—the same source as Nonius’ precious stone and the Holy Roman Emperor’s red-and-white one. The mines were worked until the late nineteenth century..." Finlay, Victoria. Jewels: A Secret History (Kindle Locations 2163–2165). Random House Publishing Group. Kindle Edition.
^Finlay, Victoria. Jewels: A Secret History (Kindle Location 1871). Random House Publishing Group. Kindle Edition.
^"Australian National Gemstone". Australian Government, Department of the Prime Minister and Cabinet. Retrieved10 June 2018.
^Graetsch, H. (1994). Heaney, P. J.; Prewitt, Connecticut; Gibbs, G. V. (eds.). "Structural characteristics of opaline and microcrystalline silica minerals. Silica, physical behavior, geochemistry and materials applications".Reviews in Mineralogy.29.
