Opal
An opal bracelet. The stone size is 18 by 15 mm (0.7 by 0.6 inches).
General
Category	Mineraloid
Chemical formula	Hydrated silica. SiO2·nH2O
Identification
Colour	White, black, red, orange, most of the full spectrum, colorless, iridescent. Very infrequently of a singular colour
Crystal habit	Irregular veins, in masses, in nodules.
Crystal system	Amorphous
Cleavage	None
Fracture	Conchoidal to uneven
Mohs Scale hardness	5.5 - 6.6
Luster	Vitreous to resinous
Refractive index	1.44 - 1.46
Pleochroism	None
Streak	White
Specific gravity	2.1 - 2.3

The mineraloid opal is amorphous SiO₂·nH₂O; hydrated silicon dioxide, the water content sometimes being as high as 20%. Opal ranges from colorless through white, milky blue, gray, red, yellow, green, brown and black. Often many of these colors can be seen at once, caused by interference and diffraction of light passing through minute, regularly arranged apertures within the microstructure of opal, known as Bragg's lattice. These apertures are filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed play of color. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal. Potch does not show a play of color. Opal bracelet, the stone is 15 mm by 18 mm. ... Opal bracelet, the stone is 15 mm by 18 mm. ... A mineraloid is a mineral-like substance that does not demonstrate crystallinity. ... A chemical formula (also called molecular formula) is a concise way of expressing information about the atoms that constitute a particular chemical compound. ... Hydrates are compounds formed by the union of water with some other substance, generally forming a neutral body, as certain crystallized salts. ... The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO2. ... The iridescence of the Blue Morpho butterfly wings. ... In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. ... Rose des Sables (Sand Rose), formed of gypsum crystals In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... Cleaveage, in mineralogy, is the tendency of crystalline materials to split along definite planes, creating smooth surfaces, of which there are several named types: Basal cleavage: cleavage parallel to the base of a crystal, or to the plane of the lateral axes. ... A fracture is the separation of a body into two, or more, pieces under the action of stress. ... A conchoidal fracture is produced when some types of fine-grained mineral, such as obsidian and flint, are broken. ... Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer. ... The refractive index of a material is the factor by which the phase velocity of electromagnetic radiation is slowed relative to vacuum. ... Pleochroism is an optical phenomenon where due to double refraction of light by a colored gem or crystal, the light is divided into two paths which are polarized at a 90° angle to each other. ... In any mineral, its streak refers to the color of its powder. ... Relative density (also known as specific gravity) is a measure of the density of a material. ... The Large Electron-Positron Collider (LEP - it was dubbed Lots of Extra Problems during the initialisation and calibration phases) is one of the largest particle accelerators finished so far. ... A mineraloid is a mineral-like substance that does not demonstrate crystallinity. ... An amorphous solid is a solid in which there is no long-range order of the positions of the atoms. ... Interference of two circular waves - Wavelength (decreasing bottom to top) and Wave centers distance (increasing to the right). ... Diffraction is the apparent bending and spreading of waves when they meet an obstruction. ... Diffraction is the apparent bending and spreading of waves when they meet an obstruction. ...

The veins of opal displaying the play of color are often quite thin, and this has given rise to unusual methods of preparing the stone as a gem. An opal doublet is a thin layer of colorful material, backed by a black mineral, such as ironstone, basalt or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch. Given the texture of opals, they can be quite difficult to polish to a reasonable lustre. The triplet cut backs the colored material with a dark backing, and then has a cap of clear quartz (rock crystal) on top, which takes a high polish, and acts as a protective layer for the comparatively delicate opal. Basalt Basalt is an extrusive igneous rock, sometimes porphyritic, and is often both fine-grained and dense. ... Top stone is obsidian, below that is pumice and in lower right hand is rhyolite (light color) Obsidian is a type of naturally occurring glass, produced from volcanoes when the right kind of lava cools rapidly, e. ... Quartz is the most abundant mineral in the Earths crust. ...

Common opal

Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish; resin opal, honey-yellow with a resinous lustre; wood opal, caused by the replacement of the organic material in wood with opal; menilite brown or grey; hyalite, a colorless glass-clear opal sometimes called Muller's Glass; geyserite, (siliceous sinter) deposited around hot springs or geysers; and diatomite or diatomaceous earth, the accumulations of diatom shells or tests. A gemstone is a mineral, rock (as in lapis lazuli) or petrified material that when cut or faceted and polished is collectible or can be used in jewellery. ... A glass of cows milk Milk most often means the nutrient fluid produced by the mammary glands of female mammals. ... Insect trapped in resin. ... A tree trunk as found at the Veluwe, The Netherlands Wood is an organic material found as the primary content of the stems of woody plants, especially trees, but also shrubs. ... Menilite is a reddish-brown form of opal, also known as liver opal or leberopal. ... Hyalite is a form of opal with a glassy appearance. ... Geyserite is a form of opaline silica that is often found around hot springs and geysers. ... Green Dragon Spring at Norris Geyser A hot spring is a place where warm or hot groundwater issues from the ground on a regular basis for at least a predictable part of the year, and is significantly above the ambient ground temperature (which is usually around 55~57°F or... Clepsydra Geyser in Yellowstone A geyser is a type of hot spring that erupts periodically, ejecting a column of hot water and steam into the air (Bryan 1995). ... Diatomaceous earth (also known as diatomite, DE or kieselguhr) is a naturally occurring, light-colored, solid material that is easily crumbled into a fine dust. ... Diatomaceous earth, also known as diatomite, kieselguhr, kieselgur, and Celite, is a naturally occurring, soft, chalk-like, sedimentary rock mineral that is easily crumbled into a fine white to off-white powder. ... Diatoms are a major group of eukaryotic algae, and are one of the most common types of phytoplankton. ...

Opal is a mineraloid gel which is deposited at relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, and basalt. The word opal comes from the Sanskrit upala, the Greek opallios, and the Latin opalus, meaning "precious stone." A gel is an apparently solid, jellylike material formed from a colloidal solution. ... Sedimentary, volcanic, plutonic, metamorphic rock types of North America. ... Limonite Limonite Limonite is a ferric hydrate of varying composition, the generic formula is frequently written as FeO(OH)·nH2O, although this is not entirely accurate as Limonite often contains a varying amount of oxide compared to hydroxide. ... Red Sandstone in Wyoming Sandstone is an arenaceous sedimentary rock composed mainly of feldspar and quartz and varies in colour (in a similar way to sand), through grey, yellow, red, and white. ... Rhyolite Rhyolite is an igneous, volcanic (extrusive) rock, of felsic composition, with aphanitic to porphyritic texture. ... Basalt Basalt is an extrusive igneous rock, sometimes porphyritic, and is often both fine-grained and dense. ... The Sanskrit language ( संस्कृता वाक्) is one of the earliest attested members of the Indo-European language family and is not only a classical language, but also an official language of India. ... Latin is the language originally spoken in the region around Rome called Latium. ...

Opal is one of the mineraloids that can form or replace fossils. The resulting fossils, though not of any extra scientific interest, appeal to collectors. A fossil Ammonite Fossils are the mineralized remains of animals or plants or other traces such as footprints. ...

Boulder opal carving of a walrus, showing flashes of colour from the exposed opal. The carving is 9 cm (3.5 inches) long.

Download high resolution version (800x648, 119 KB)A boulder opal carving of a walrus. ... Download high resolution version (800x648, 119 KB)A boulder opal carving of a walrus. ...

Sources of opal

A large fraction of the world's opal comes from Australia. The town of Coober Pedy, in particular, is a major source. Common, water, jelly, and fire opal are found mostly in Mexico and Mesoamerica. The main source of opal in the United States is Spencer, Idaho. Lightning Ridge in Australia is the main source of black opal, opal containing a predominantly dark background (dark-gray to blue-black displaying the play of color). Coober Pedy, population 3,500, is a small town in South Australia, 846 kilometres north of Adelaide on the Stuart Highway. ... Mesoamerica is the region extending from central Mexico south to the northwestern border of Costa Rica that gave rise to a group of stratified, culturally related agrarian civilizations spanning an approximately 3,000-year period before the African discovery of the New World by Columbus. ... Spencer is a city located in Clark County, Idaho. ...

Synthetic opal

As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of colour are seen to be arranged in a "lizard skin" or "chicken wire" pattern. Synthetics are further distinguished from naturals by the former's lack of fluorescence under UV light. Synthetics are also generally lower in density and are often highly porous; some may even stick to the tongue. Fluorescence induced by exposure to ultraviolet light in vials containing various sized cadmium selenide (CdSe) quantum dots. ... Note: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. ...

Two notable producers of synthetic opal are the companies Kyocera and Inamori of Japan. Most so-called synthetics, however, are more correctly termed imitations, as they contain substances not found in natural opal (e.g., plastic stabilizers). The Gilson opals often seen in vintage jewellery are actually an imitation consisting of laminated glass with bits of foil interspersed. Kyocera (京セラ, also known as Kyoto Ceramics) is a Japanese company based in Kyoto, Japan. ...

The opal is the official gemstone of South Australia. Opal is the official birthstone of the month of October. A gemstone is a mineral, rock (as in lapis lazuli) or petrified material that when cut or faceted and polished is collectible or can be used in jewellery. ... Motto: United for the Common Wealth Nickname: Festival State Other Australian states and territories Capital Adelaide Government Governor Premier Const. ... A birthstone is a gemstone or other semi-precious stone which is associated with a month of the Gregorian Calendar. ... October is the tenth month of the year in the Gregorian Calendar and one of seven Gregorian months with the length of 31 days. ...

See also

• Isabella Rigney
• List of minerals
• Optical phenomena

Isabella Rigney, an Australian pioneer, born in Poland on the 22nd February 1942 is an acclaimed opal miner of Australia. ... Gem animals. ... An optical phenomenon is any observable event which results from the interaction of light and matter. ...

References and external links

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0471805807
• Mineral Galleries - Opal
• Mindat.org - Opal
• Minerals.net - Opal
• Berkeley - Great photos
• Gem gallery - Opal