opal

/oh"peuhl/, n.

1. a mineral, an amorphous form of silica, SiO₂ with some water of hydration, found in many varieties and colors, including a form that is milky white.

2. an iridescent variety of this that is used as a gem.

3. a gem of this.

[1350-1400; ME < L opalus < Gk opállios opal, gem; prob. from a source akin to Skt upala precious stone]

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A hydrated, noncrystalline silica mineral used extensively as a gemstone.

Its chemical composition is similar to that of quartz but generally with a variable water content. Pure opal is colourless, but impurities generally give it various dull colours ranging from yellow and red to black. Black opal is especially rare and valuable. White opal and fire opal, characterized by yellow, orange, or red colour, are much more common. Various forms of common opal are widely used as abrasives, insulation material, and ceramic ingredients. Opal is most abundant in volcanic rocks, especially in areas of hot-spring activity. The finest gem opals have been found in Australia; other areas that yield gem material include Japan, Mexico, Honduras, India, New Zealand, and the U.S.

Black opal from Australia; in the collection of the Department of Earth Sciences, Washington ...

John H. Gerard

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▪ mineral

 silica mineral extensively used as a gemstone, a submicrocrystalline variety of cristobalite (q.v.). In ancient times opal was included among the noble gems and was ranked second only to emerald by the Romans. In the Middle Ages it was supposed to be lucky, but in modern times it has been regarded as unlucky.

      Opal is fundamentally colourless, but such material is rarely found. Disseminated impurities generally impart to opal various dull body colours that range from the yellows and reds derived from iron oxides to black from manganese oxides and organic carbon. The milkiness of many white and gray opals is attributable to an abundance of tiny gas-filled cavities in them. Black opal, with a very dark gray or blue to black body colour, is particularly rare and highly prized. White opal, with light body colours, and fire opal, characterized by yellow, orange, or red body colour, are much more common.

      Precious opals are translucent to transparent and are distinguished by a combination of milky to pearly opalescence and an attractive play of many colours. These colours flash and change as a stone is viewed from different directions and are caused by interference of light along minute cracks and other internal inhomogeneities.

      Opal is deposited from circulating waters in such varied forms as nodules, stalactitic masses, veinlets, and encrustations and is widely distributed in nearly all kinds of rocks. It is most abundant in volcanic rocks, especially in areas of hot-spring activity. It also forms pseudomorphs after wood and other fossil organic matter and after gypsum, calcite, feldspars, and many other minerals that it has replaced. As the siliceous material secreted by organisms such as diatoms and radiolarians, opal constitutes important parts of many sedimentary accumulations.

      The finest gem opals have been obtained from South Australia, Queensland, and New South Wales in Australia; the Lightning Ridge field is famous for superb black stones. Deposits of white opal in Japan, fire opal in Mexico and Honduras, and several varieties of precious opal in India, New Zealand, and the western United States also have yielded much gem material. Most of the precious opal marketed in ancient times was obtained from occurrences in what is now Slovakia. Various forms of common opal are widely mined for use as abrasives, insulation media, fillers, and ceramic ingredients.

      Fire opals usually are facet cut, but most other precious opals are finished en cabochon (cabochon cut) because their optical properties are best displayed on smoothly rounded surfaces. Undersized fragments are used for inlay work, and small pieces scattered throughout a natural matrix are commonly sold under the name root of opal. Because opal may crack or lose its colour if it dries, many finished stones are protected by water or films of oil until they are sold. Opals absorb liquids very readily. An extremely porous variety, known as hydrophane, can absorb surprising quantities of water; it is almost opaque when dry but nearly transparent when saturated. Light-coloured stones are often dyed (dye) to resemble rarer, more deeply coloured varieties.

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