Opalsilica mineral extensively used as a gemstonea submicrocrystalline variety of cristobalite. 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 Opal - Sound Of Stereo - Mineral 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 Jagged Edge - Fuel For Your Soul 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 Fumblebee - David Grisman - Dawg Jazz / Dawg Grass 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 AustraliaQueensland, 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 Opal - Sound Of Stereo - Mineral India, New Zealandand the western United States also Dead Mind - Boon - The Almighty Love 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 Opal - Sound Of Stereo - Mineral 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 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 Plutoman - Inspiral Carpets - Devil Hopping hydrophane, can absorb surprising quantities of water; it is almost opaque when dry but nearly transparent when saturated.
Light-coloured stones are often dyed to resemble rarer, more deeply coloured varieties. Article Media. Info Print Cite. Submit Feedback. Thank you for your feedback. Opal mineral. Read More on This Topic. Opal is poorly crystalline or amorphous hydrous silica that is compact and vitreous and most commonly translucent white….
Subscribe Today. Learn More in these related Britannica articles:. Opal is poorly crystalline or amorphous hydrous silica that is compact and vitreous and most commonly translucent white to colourless.
Precious opal reflects light with a play of brilliant colours across the visible spectrum, red being the most valued. Opal forms by precipitation from…. History at your fingertips. Sign up here to see what happened On This Dayevery day in your inbox! By signing up, you agree to our Privacy Notice. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox.
More About. Opal - Sound Of Stereo - Mineral from Britannica Encyclopedias for elementary and high school students.
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