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.
99-529: Opal is a hydrated amorphous form of silica (SiO 2 · n H 2 O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous chemical structure, it is classified as a mineraloid , unlike crystalline forms of silica, which are considered 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 . The name opal
198-473: A druse (a layer of crystals lining a void), of which quartz geodes are particularly fine examples. The crystals are attached at one end to the enclosing rock, and only one termination pyramid is present. However, doubly terminated crystals do occur where they develop freely without attachment, for instance, within gypsum . α-quartz crystallizes in the trigonal crystal system, space group P 3 1 21 or P 3 2 21 (space group 152 or 154 resp.) depending on
297-560: A pressure cooker . However, the quality and size of the crystals that were produced by these early efforts were poor. Elemental impurity incorporation strongly influences the ability to process and utilize quartz. Naturally occurring quartz crystals of extremely high purity, necessary for the crucibles and other equipment used for growing silicon wafers in the semiconductor industry, are expensive and rare. These high-purity quartz are defined as containing less than 50 ppm of impurity elements. A major mining location for high purity quartz
396-485: 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 . Opal is considered the birthstone for people born in October. Hydrate Hydrates are inorganic salts "containing water molecules combined in a definite ratio as an integral part of the crystal " that are either bound to a metal center or that have crystallized with
495-663: A burial context, such as Newgrange or Carrowmore in Ireland . Quartz was also used in Prehistoric Ireland , as well as many other countries, for stone tools ; both vein quartz and rock crystal were knapped as part of the lithic technology of the prehistoric peoples. While jade has been since earliest times the most prized semi-precious stone for carving in East Asia and Pre-Columbian America, in Europe and
594-472: A continuous framework of SiO 4 silicon–oxygen tetrahedra , with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO 2 . Quartz is, therefore, classified structurally as a framework silicate mineral and compositionally as an oxide mineral . Quartz is the second most abundant mineral in Earth 's continental crust , behind feldspar . Quartz exists in two forms,
693-487: A defining constituent of granite and other felsic igneous rocks . It is very common in sedimentary rocks such as sandstone and shale . It is a common constituent of schist , gneiss , quartzite and other metamorphic rocks . Quartz has the lowest potential for weathering in the Goldich dissolution series and consequently it is very common as a residual mineral in stream sediments and residual soils . Generally
792-436: 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. 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
891-402: A form of clathrate . An important example is methane hydrate (also known as gas hydrate, methane clathrate, etc.). Nonpolar molecules such as methane can form clathrate hydrates with water, especially under high pressure. Although there is no hydrogen bonding between water and guest molecules when methane is the guest molecule of the clathrate, guest–host hydrogen bonding often forms when
990-464: A fresh bay leaf and held in the hand. As a result, the opal was seen as the patron gemstone for thieves during the medieval period . 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,
1089-403: 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. "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,
SECTION 10
#17327729094211188-589: 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. Combined with modern techniques of polishing,
1287-584: A high presence of quartz suggests a " mature " rock, since it indicates the rock has been heavily reworked and quartz was the primary mineral that endured heavy weathering. While the majority of quartz crystallizes from molten magma , quartz also chemically precipitates from hot hydrothermal veins as gangue , sometimes with ore minerals like gold, silver and copper. Large crystals of quartz are found in magmatic pegmatites . Well-formed crystals may reach several meters in length and weigh hundreds of kilograms. The largest documented single crystal of quartz
1386-584: A molecular entity". For example: ethanol , CH 3 −CH 2 −OH , is the product of the hydration reaction of ethene , CH 2 =CH 2 , formed by the addition of H to one C and OH to the other C, and so can be considered as the hydrate of ethene. A molecule of water may be eliminated, for example, by the action of sulfuric acid . Another example is chloral hydrate , CCl 3 −CH(OH) 2 , which can be formed by reaction of water with chloral , CCl 3 −CH=O . Many organic molecules, as well as inorganic molecules, form crystals that incorporate water into
1485-405: 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
1584-574: A small Brazilian mine, but it is also seen in Lower Silesia in Poland . Naturally occurring prasiolite is also found in the Thunder Bay area of Canada . Quartz crystals have piezoelectric properties; they develop an electric potential upon the application of mechanical stress . Quartz's piezoelectric properties were discovered by Jacques and Pierre Curie in 1880. Quartz is
1683-404: 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 of inclusions and
1782-437: A submicroscopic distribution of colloidal ferric hydroxide impurities. Natural citrines are rare; most commercial citrines are heat-treated amethysts or smoky quartzes . However, a heat-treated amethyst will have small lines in the crystal, as opposed to a natural citrine's cloudy or smoky appearance. It is nearly impossible to differentiate between cut citrine and yellow topaz visually, but they differ in hardness . Brazil
1881-680: A superstition that it would bring prosperity. Citrine was first appreciated as a golden-yellow gemstone in Greece between 300 and 150 BC, during the Hellenistic Age . Yellow quartz was used prior to that to decorate jewelry and tools but it was not highly sought after. Milk quartz or milky quartz is the most common variety of crystalline quartz. The white color is caused by minute fluid inclusions of gas, liquid, or both, trapped during crystal formation, making it of little value for optical and quality gemstone applications. Rose quartz
1980-577: 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 Wegeltena , 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. Wollo Province opal, more commonly referred to as "Welo" or "Wello" opal, has become
2079-527: 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 of Querétaro in Mexico; these opals are commonly called Mexican fire opals. 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
SECTION 20
#17327729094212178-511: Is a form of opal, as well as a type 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 . 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
2277-542: Is a form of quartz that ranges from a bright vivid violet to a dark or dull lavender shade. The world's largest deposits of amethysts can be found in Brazil, Mexico, Uruguay, Russia, France, Namibia, and Morocco. Sometimes amethyst and citrine are found growing in the same crystal. It is then referred to as ametrine . Amethyst derives its color from traces of iron in its structure. Blue quartz contains inclusions of fibrous magnesio-riebeckite or crocidolite . Inclusions of
2376-720: 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 + 3 ⁄ 4 in (120 mm) and a width of 4 + 1 ⁄ 2 in (110 mm). The Mintabie Opal Field in South Australia located about 250 km (160 mi) northwest of Coober Pedy has also produced large quantities of crystal opal and
2475-419: Is a monoclinic polymorph. Lechatelierite is an amorphous silica glass SiO 2 which is formed by lightning strikes in quartz sand . As quartz is a form of silica, it is a possible cause for concern in various workplaces. Cutting, grinding, chipping, sanding, drilling, and polishing natural and manufactured stone products can release hazardous levels of very small, crystalline silica dust particles into
2574-463: Is a primary identifier for the macrocrystalline varieties. Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent or translucent and has often been used for hardstone carvings , such as the Lothair Crystal . Common colored varieties include citrine, rose quartz, amethyst, smoky quartz, milky quartz, and others. These color differentiations arise from
2673-419: 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 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
2772-707: 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 from Oregon and Idaho in the Owyhee region, as well as from Nevada around the Virgin Valley . Opal is also formed by diatoms . 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 them structural coloration and therefore
2871-400: Is a type of quartz that exhibits a pale pink to rose red hue. The color is usually considered as due to trace amounts of titanium , iron , or manganese in the material. Some rose quartz contains microscopic rutile needles that produce asterism in transmitted light. Recent X-ray diffraction studies suggest that the color is due to thin microscopic fibers of possibly dumortierite within
2970-613: Is adapted from the Latin term opalus . The origin of this word in turn is a matter of debate, but most modern references suggest it is adapted from the Sanskrit word úpala meaning ‘precious stone’. As references to the gem are made by Pliny the Elder , one theory attributed the name's origin to Roman mythology: to have been adapted from Ops , the wife of Saturn , and goddess of fertility. (The portion of Saturnalia devoted to Ops
3069-614: Is believed to be derived from the Sanskrit word upala ( उपल ), which means 'jewel', and later the Greek derivative opállios ( ὀπάλλιος ). There are two broad classes of opal: precious and common. Precious opal displays play-of-color ( iridescence ); common opal does not. 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." The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Depending on
Opal - Misplaced Pages Continue
3168-540: 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
3267-416: Is composed of silica spheres some 150–300 nm (5.9 × 10–1.18 × 10 in) in diameter in a hexagonal or cubic close-packed lattice . It was shown by J. V. Sanders in the mid-1960s that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal. The regularity of the sizes and the packing of these spheres
3366-537: 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, Royal Peacock, Bonanza, Opal Queen, and WRT Stonetree/Black Beauty mines. The largest unpolished black opal in
3465-596: 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. 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
3564-410: 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. Opal occurs in significant quantity and variety in central Mexico, where mining and production first originated in the state of Querétaro . In this region
3663-427: 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 diatomaceous earth , the accumulations of diatom shells or tests. Common opal often displays
3762-523: Is often twinned , synthetic quartz is produced for use in industry. Large, flawless, single crystals are synthesized in an autoclave via the hydrothermal process . Like other crystals, quartz may be coated with metal vapors to give it an attractive sheen. Quartz is the most common material identified as the mystical substance maban in Australian Aboriginal mythology . It is found regularly in passage tomb cemeteries in Europe in
3861-401: 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
3960-434: 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. The surface of opal in contact with water is covered by siloxane bonds (≡Si–O–Si≡) and silanol groups (≡Si–OH). This makes the opal surface very hydrophilic and capable of forming numerous hydrogen bonds . The word 'opal'
4059-468: Is 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. Dehydration experiments and infrared spectroscopy have shown that most of
Opal - Misplaced Pages Continue
4158-485: 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 . For gemstone use, most opal is cut and polished to form a cabochon . "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 opal doublet consists of
4257-729: Is the Spruce Pine Gem Mine in Spruce Pine, North Carolina , United States. Quartz may also be found in Caldoveiro Peak , in Asturias , Spain. By the 1930s, the electronics industry had become dependent on quartz crystals. The only source of suitable crystals was Brazil; however, World War II disrupted the supplies from Brazil, so nations attempted to synthesize quartz on a commercial scale. German mineralogist Richard Nacken (1884–1971) achieved some success during
4356-475: Is the leading producer of citrine, with much of its production coming from the state of Rio Grande do Sul . The name is derived from the Latin word citrina which means "yellow" and is also the origin of the word " citron ". Sometimes citrine and amethyst can be found together in the same crystal, which is then referred to as ametrine . Citrine has been referred to as the "merchant's stone" or "money stone", due to
4455-413: Is the number of water molecules per formula unit of the salt, is commonly used to show that a salt is hydrated. The n is usually a low integer , though it is possible for fractional values to occur. For example, in a monohydrate n = 1, and in a hexahydrate n = 6. Numerical prefixes mostly of Greek origin are: A hydrate that has lost water is referred to as an anhydride ;
4554-412: Is used for very accurate measurements of very small mass changes in the quartz crystal microbalance and in thin-film thickness monitors . Almost all the industrial demand for quartz crystal (used primarily in electronics) is met with synthetic quartz produced by the hydrothermal process. However, synthetic crystals are less prized for use as gemstones. The popularity of crystal healing has increased
4653-524: The hexagonal crystal system above 573 °C (846 K; 1,063 °F). The ideal crystal shape is a six-sided prism terminating with six-sided pyramid-like rhombohedrons at each end. In nature, quartz crystals are often twinned (with twin right-handed and left-handed quartz crystals), distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive . Well-formed crystals typically form as
4752-879: The 14th century in Middle High German and in East Central German and which came from the Polish dialect term twardy , which corresponds to the Czech term tvrdý ("hard"). Some sources, however, attribute the word's origin to the Saxon word Querkluftertz , meaning cross-vein ore . The Ancient Greeks referred to quartz as κρύσταλλος ( krustallos ) derived from the Ancient Greek κρύος ( kruos ) meaning "icy cold", because some philosophers (including Theophrastus ) understood
4851-620: The 1930s and 1940s. After the war, many laboratories attempted to grow large quartz crystals. In the United States, the U.S. Army Signal Corps contracted with Bell Laboratories and with the Brush Development Company of Cleveland, Ohio to synthesize crystals following Nacken's lead. (Prior to World War II, Brush Development produced piezoelectric crystals for record players.) By 1948, Brush Development had grown crystals that were 1.5 inches (3.8 cm) in diameter,
4950-457: The Earth's crust exposed to high temperatures, thereby damaging materials containing quartz and degrading their physical and mechanical properties. Although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is a secondary identifier for the cryptocrystalline minerals, although it
5049-527: The H 2 O in the formula of SiO 2 · n H 2 O 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 structure of low-pressure polymorphs of anhydrous silica consists of frameworks of fully corner bonded tetrahedra of SiO 4 . The higher temperature polymorphs of silica cristobalite and tridymite are frequently
SECTION 50
#17327729094215148-466: The Middle East the different varieties of quartz were the most commonly used for the various types of jewelry and hardstone carving , including engraved gems and cameo gems , rock crystal vases , and extravagant vessels. The tradition continued to produce objects that were very highly valued until the mid-19th century, when it largely fell from fashion except in jewelry. Cameo technique exploits
5247-745: 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 (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". Fire opal is
5346-576: The absence of any surrounding non-opal matrix . 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. Synthetic opals are also deeply investigated in photonics for sensing and light management purposes. The lattice of spheres of opal that cause interference with light
5445-441: The air that workers breathe. Crystalline silica of respirable size is a recognized human carcinogen and may lead to other diseases of the lungs such as silicosis and pulmonary fibrosis . Not all varieties of quartz are naturally occurring. Some clear quartz crystals can be treated using heat or gamma-irradiation to induce color where it would not otherwise have occurred naturally. Susceptibility to such treatments depends on
5544-433: 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. 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
5643-407: The bands of color in onyx and other varieties. Efforts to synthesize quartz began in the mid-nineteenth century as scientists attempted to create minerals under laboratory conditions that mimicked the conditions in which the minerals formed in nature: German geologist Karl Emil von Schafhäutl (1803–1890) was the first person to synthesize quartz when in 1845 he created microscopic quartz crystals in
5742-444: The chirality. Above 573 °C (846 K; 1,063 °F), α-quartz in P 3 1 21 becomes the more symmetric hexagonal P 6 4 22 (space group 181), and α-quartz in P 3 2 21 goes to space group P 6 2 22 (no. 180). These space groups are truly chiral (they each belong to the 11 enantiomorphous pairs). Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks (SiO 4 tetrahedra in
5841-427: 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
5940-466: The crystal structure. Prase is a green variety of quartz. The green color is caused by inclusions of amphibole . Prasiolite , also known as vermarine , is a variety of quartz that is green in color. The green is caused by iron ions. It is a rare mineral in nature and is typically found with amethyst; most "prasiolite" is not natural – it has been artificially produced by heating of amethyst. Since 1950 , almost all natural prasiolite has come from
6039-437: The crystalline structure without chemical alteration of the organic molecule ( water of crystallization ). The sugar trehalose , for example, exists in both an anhydrous form ( melting point 203 °C) and as a dihydrate (melting point 97 °C). Protein crystals commonly have as much as 50% water content. Molecules are also labeled as hydrates for historical reasons not covered above. Glucose , C 6 H 12 O 6 ,
SECTION 60
#17327729094216138-433: The crystals. Tridymite and cristobalite are high-temperature polymorphs of SiO 2 that occur in high-silica volcanic rocks. Coesite is a denser polymorph of SiO 2 found in some meteorite impact sites and in metamorphic rocks formed at pressures greater than those typical of the Earth's crust. Stishovite is a yet denser and higher-pressure polymorph of SiO 2 found in some meteorite impact sites. Moganite
6237-417: The dominant Ethiopian opal in the gem trade. 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 in situ within a subsurface horizon or zone of bentonite , which
6336-637: 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
6435-778: 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 or Opal-CT has been interpreted as consisting of clusters of stacked cristobalite and tridymite 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
6534-469: 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. It was also said to grant invisibility if wrapped in
6633-551: 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, with the state of South Australia accounting for 80% of the world's supply. In 2012, Ethiopian opal production was estimated to be 14,000 kg (31,000 lb) by the United States Geological Survey. USGS data from the same period (2012), reveals Australian opal production to be $ 41 million. Because of
6732-479: The guest is a larger organic molecule such as tetrahydrofuran . In such cases the guest–host hydrogen bonds result in the formation of L-type Bjerrum defects in the clathrate lattice. The stability of hydrates is generally determined by the nature of the compounds, their temperature, and the relative humidity (if they are exposed to air). Quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide ). The atoms are linked in
6831-442: The hands. This idea persisted until at least the 17th century. He also knew of the ability of quartz to split light into a spectrum . In the 17th century, Nicolas Steno 's study of quartz paved the way for modern crystallography . He discovered that regardless of a quartz crystal's size or shape, its long prism faces always joined at a perfect 60° angle. Quartz belongs to the trigonal crystal system at room temperature, and to
6930-437: The impurities of phosphate and aluminium that formed crystalline rose quartz, unlike the iron and microscopic dumortierite fibers that formed rose quartz. Smoky quartz is a gray, translucent version of quartz. It ranges in clarity from almost complete transparency to a brownish-gray crystal that is almost opaque. Some can also be black. The translucency results from natural irradiation acting on minute traces of aluminum in
7029-454: The incident light. The process can be described by Bragg's law of diffraction. Visible light cannot pass through large thicknesses of the opal. This is the basis of the optical band gap in a photonic crystal . In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during its formation. The term opalescence is commonly used to describe this unique and beautiful phenomenon, which in gemology
7128-452: The largest at that time. By the 1950s, hydrothermal synthesis techniques were producing synthetic quartz crystals on an industrial scale, and today virtually all the quartz crystal used in the modern electronics industry is synthetic. An early use of the piezoelectricity of quartz crystals was in phonograph pickups. One of the most common piezoelectric uses of quartz today is as a crystal oscillator . The quartz oscillator or resonator
7227-409: The literature. Water content may be as high as 10 wt%. Opal-CT, 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%. Two broad categories of noncrystalline opals, sometimes just referred to as "opal-A" ("A" stands for "amorphous"), have been proposed. The first of these
7326-517: The location from which the quartz was mined. Prasiolite, an olive colored material, is produced by heat treatment; natural prasiolite has also been observed in Lower Silesia in Poland. Although citrine occurs naturally, the majority is the result of heat-treating amethyst or smoky quartz. Carnelian has been heat-treated to deepen its color since prehistoric times. Because natural quartz
7425-476: The metal complex. Such hydrates are also said to contain water of crystallization or water of hydration . If the water is heavy water in which the constituent hydrogen is the isotope deuterium , then the term deuterate may be used in place of hydrate . A colorful example is cobalt(II) chloride , which turns from blue to red upon hydration , and can therefore be used as a water indicator. The notation " hydrated compound ⋅ n H 2 O ", where n
7524-421: The mineral dumortierite within quartz pieces often result in silky-appearing splotches with a blue hue. Shades of purple or gray sometimes also are present. "Dumortierite quartz" (sometimes called "blue quartz") will sometimes feature contrasting light and dark color zones across the material. "Blue quartz" is a minor gemstone. Citrine is a variety of quartz whose color ranges from pale yellow to brown due to
7623-519: The mineral to be a form of supercooled ice. Today, the term rock crystal is sometimes used as an alternative name for transparent coarsely crystalline quartz. Roman naturalist Pliny the Elder believed quartz to be water ice , permanently frozen after great lengths of time. He supported this idea by saying that quartz is found near glaciers in the Alps, but not on volcanic mountains, and that large quartz crystals were fashioned into spheres to cool
7722-501: The most commonly used minerals in the making of jewelry and hardstone carvings , especially in Europe and Asia. Quartz is the mineral defining the value of 7 on the Mohs scale of hardness , a qualitative scratch method for determining the hardness of a material to abrasion. The word "quartz" is derived from the German word Quarz , which had the same form in the first half of
7821-512: The normal α-quartz and the high-temperature β-quartz, both of which are chiral . The transformation from α-quartz to β-quartz takes place abruptly at 573 °C (846 K; 1,063 °F). Since the transformation is accompanied by a significant change in volume, it can easily induce microfracturing of ceramics or rocks passing through this temperature threshold. There are many different varieties of quartz, several of which are classified as gemstones . Since antiquity, varieties of quartz have been
7920-474: 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 orange-red background color
8019-577: 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 ), Honduras (more precisely in Erandique ), Guatemala, and Nicaragua. In late 2008, NASA announced the discovery of opal deposits on Mars . Wood opal , also known as xylopal ,
8118-462: 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 20 years or so. Even as recently as the beginning of the 20th century, it was believed that when
8217-478: 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 . Another source of white base opal or creamy opal in the United States is Spencer, Idaho . A high percentage of
8316-782: The play of color), collected from the Griman Creek Formation . 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 opalized fossil remains, including dinosaur bones in New South Wales and South Australia, and marine creatures in South Australia. It has been reported that Northern African opal
8415-494: The presence of impurities which change the molecular orbitals, causing some electronic transitions to take place in the visible spectrum causing colors. The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties ( aggregates of crystals visible only under high magnification). The cryptocrystalline varieties are either translucent or mostly opaque, while
8514-403: The present case). The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without a change in the way they are linked. However, there is a significant change in volume during this transition, and this can result in significant microfracturing in ceramics during firing, in ornamental stone after a fire and in rocks of
8613-510: The quartz. Additionally, there is a rare type of pink quartz (also frequently called crystalline rose quartz) with color that is thought to be caused by trace amounts of phosphate or aluminium . The color in crystals is apparently photosensitive and subject to fading. The first crystals were found in a pegmatite found near Rumford , Maine , US, and in Minas Gerais , Brazil. The crystals found are more transparent and euhedral, due to
8712-579: 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
8811-434: The remaining water, if any exists, can only be removed with very strong heating. A substance that does not contain any water is referred to as anhydrous . Some anhydrous compounds are hydrated so easily that they are said to be hygroscopic and are used as drying agents or desiccants . In organic chemistry, a hydrate is a compound formed by the hydration, i.e. "Addition of water or of the elements of water (i.e. H and OH) to
8910-655: 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 lesser quality) are Guerrero , which produces an opaque opal similar to
9009-414: The transparent varieties tend to be macrocrystalline. Chalcedony is a cryptocrystalline form of silica consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite . Other opaque gemstone varieties of quartz, or mixed rocks including quartz, often including contrasting bands or patterns of color, are agate , carnelian or sard, onyx , heliotrope , and jasper . Amethyst
9108-422: The true girasol opal 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. A Peruvian opal (also called blue opal)
9207-599: 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. Yet, the validity of data in the USGS report appears to conflict with that of Laurs et al. and Mesfin, who estimated the 2012 Ethiopian opal output (from Wegeltena ) to be only 750 kg (1,650 lb). The town of Coober Pedy in South Australia
9306-470: Was "Opalia", similar to opalus .) Another common claim was that the term was 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
9405-493: Was a gem prized by royalty. Until the opening of vast deposits in Australia in the 19th century the only known source was Červenica beyond the Roman frontier in Slovakia . Opal is the national gemstone of Australia. 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
9504-420: Was first developed by Walter Guyton Cady in 1921. George Washington Pierce designed and patented quartz crystal oscillators in 1923. The quartz clock is a familiar device using the mineral. Warren Marrison created the first quartz oscillator clock based on the work of Cady and Pierce in 1927. The resonant frequency of a quartz crystal oscillator is changed by mechanically loading it, and this principle
9603-541: Was found near Itapore , Goiaz , Brazil; it measured approximately 6.1 m × 1.5 m × 1.5 m (20 ft × 5 ft × 5 ft) and weighed over 39,900 kg (88,000 lb). Quartz is extracted from open pit mines . Miners occasionally use explosives to expose deep pockets of quartz. More frequently, bulldozers and backhoes are used to remove soil and clay and expose quartz veins, which are then worked using hand tools. Care must be taken to avoid sudden temperature changes that may damage
9702-590: Was originally thought of as C 6 (H 2 O) 6 and described as a carbohydrate . Hydrate formation is common for active ingredients . Many manufacturing processes provide an opportunity for hydrates to form and the state of hydration can be changed with environmental humidity and time. The state of hydration of an active pharmaceutical ingredient can significantly affect the solubility and dissolution rate and therefore its bioavailability . Clathrate hydrates (also known as gas hydrates, gas clathrates, etc.) are water ice with gas molecules trapped within; they are
9801-474: 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. The opal, found mostly in the form of nodules, was of volcanic origin and was found predominantly within weathered layers of rhyolite . This Shewa Province opal was mostly dark brown in color and had
#420579