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OPAL

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Originally appearing in Volume V20, Page 121 of the 1911 Encyclopedia Britannica.
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OPAL , an amorphous or non-crystalline See also:

mineral consisting of hydrated See also:silica, occasionally displaying a beautiful See also:play of See also:colour, whence its value as a See also:gem-See also:stone. It is named from See also:Lat. opal us, Gr. &iraXkov, with which may be compared Sansk. upala, a See also:precious stone. Opal commonly occurs in nodular or stalactitic masses, in the cavities of volcanic rocks, having been deposited in a gelatinous or colloidal See also:condition. It is inferior to See also:quartz in hardness (H. 5.5 to 6.5) and in See also:density (S. G. 1.9 to 2.3), whilst it differs also by its solubility in See also:caustic alkalis. The proportion of See also:water in opal varies usually from 3 to 12%, and it is said that occasionally no water can be detected, the mineral having apparently suffered dehydration. Though normally isotropic, opal is frequently doubly refracting, the See also:anomaly being due to tension set up during consolidation. The mineral when pure is transparent and colourless, as well seen in the variety which, from its vitreous See also:appearance, was called by A. G.

See also:

Werner hyalite (Gr. uaAoc, See also:glass), or popularly "See also:Miller's glass," a name said to have been taken from its discoverer. This pellucid opaline silica occurs as an incrustation in small globules, and is by no means a See also:common mineral, being chiefly found at certain localities in Bohemia, See also:Mexico and See also:Colorado, U.S.A. (Cripple See also:Creek). The beautiful variety known as " See also:noble " or " precious opal " owes its value to the brilliant flashes of colour which it displays by reflected See also:light. The See also:colours are not due to the presence of any material pigment, but result from certain structural peculiarities in the stone, perhaps from microscopic fissures or pores or from delicate striae, but more probably from very thin lamellae of See also:foreign See also:matter, or of opaline silica, having a different See also:index of See also:refraction from that of the See also:matrix. The origin of the colours in opal has been studied by See also:Sir D. See also:Brewster, Sir W. See also:Crookes, See also:Lord See also:Rayleigh and H. Behrens. In the variety known to jewellers as " See also:harlequin opal," the See also:rainbow-like tints are flashed forth from small angular surfaces, forming a See also:kind of polychromatic See also:mosaic, whilst in other varieties the colours are disposed in broad bands or irregular patches of comparatively large See also:area. By moving the stone, a brilliant See also:succession of fiery flashes may sometimes be obtained. The opal is usually cut with a See also:convex See also:surface, and, being a soft stone, should be protected from See also:friction likely to produce See also:abrasion; nor should it be exposed to sudden alternations of temperature.

The loss of water, sometimes effected by See also:

heat, greatly impairs the colour, though moderate warmth may improve it. According to See also:Pliny the opal ranked next in value to the See also:emerald, and he relates that the See also:rich See also:Roman senator Nonius was exiled by See also:Mark Antony for See also:sake of his magnificent opal, as large as a See also:hazel See also:nut. The opal, on See also:account of its unique characters, has been the subject of remarkable superstition, and even in See also:modern times has often been regarded as an unlucky stone, but in See also:recent years it has regained popular favour and is now when See also:fine, among the most highly valued gem-stones. Precious opal is a mineral of very limited See also:distribution. Though See also:ancient writers See also:state that it was brought from See also:India, and fine stones are still called in See also:trade " See also:Oriental opal," its occurrence is not known in the See also:East. The finest opals seem to have been always obtained from See also:Hungary, where the mineral occurs, associated with much common opal, in nests in an altered andesitic See also:rock. The fine opals occur only at the Dubnyik mine, near the See also:village of Vorosvagras (Czerwenitza). The workings have been carried on for centuries in the mountains near See also:Eperjes, and some remarkable stones from this locality are preserved in the Imperial Natural See also:History Museum in See also:Vienna, including an uncut specimen weighing about 3000 carats. Precious opal is found also in See also:Honduras, especially in See also:trachyte near Graciasa Dios; and in Mexico, where it occurs in a porphyritic rock at Esperanza ih the state of See also:Queretaro. A remarkable kind of opal, of yellow or See also:hyacinth-red colour, occurs in trachytic See also:porphyry at Zimapan in See also:Hidalgo, Mexico, and is known as " See also:fire-opal." This variety is not only cut en cabochon but is also faceted. Fire-opal is sometimes called " girasol." Much precious opal is worked in See also:Australia. In See also:Queensland it is found lining cracks in nodules of See also:brown ironstone in the See also:Desert See also:Sand-stone, a rock of Upper Cretaceous See also:age, and is distributed over a wide area near the Barcoo See also:river.

Bulla Creek is a well-known locality. The layer of opal, when too thin to be cut with a convex surface, is used for inlaid See also:

work or is carved into cameos which show to much See also:advantage against the dark-brown matrix. The matrix penetrated by See also:veins and spots of opal, and perhaps heightened in colour artificially, has been called " See also:black opal "; but true black opal occurs in New See also:South See also:Wales. The " See also:root of opal " consists of the mineral disseminated through the matrix. In New South Wales precious opal was accidentally discovered in 1889, and is now largely worked at See also:White Cliffs, Yungnulgra See also:county, where it is found in nodules and seams in a siliceous rock of the Upper Cretaceous See also:series. It is notable that the opal sometimes replaces shells and even reptilian bones, whilst curious pseudomorphs, known as " pineapple opal," show the opal in the See also:form of aggregated crystals, perhaps of See also:gypsum, gaylussite or glauberite. " Common opal " is the name generally applied to the varieties which exhibit no beauty of colour, and may be nearly opaque. It is frequently found in the vesicular lavas of the N.E. of See also:Ireland, the See also:west of See also:Scotland, the Faroe Isles and See also:Iceland. When of milky-white colour it is known as " See also:milk opal "; when of resinous and waxy appearance as " See also:resin opal "; if banded it is called " See also:agate opal "; a See also:green variety is termed " prase opal "; a dark red, ferruginous variety " jaspar opal "; whilst " See also:rose opal " is a beautiful See also:pink mineral, coloured with organic matter, found at See also:Quincy, near Mehun-sur-Yevre, in See also:France. A brown or See also:grey concretionary opal from See also:Tertiary shales at Menilmontant, near See also:Paris, is known as menilite or " See also:liver opal." A dull opaque form of opal, with a fracture imperfectly conchoidal, is called " semi-opal "; whilst the opal which not infrequently forms the mineralizing substance of fossil See also:wood passes as " wood opal." The name hydrophane is applied to a porous opal, perhaps partially dehydrated, which is almost opaque when dry but becomes more or less transparent when immersed in water. It has been sometimes sold in See also:America as " magic stone." Cacholong is another kind of porous opal with a lustre rather like that of See also:mother-of-See also:pearl, said to have been named from the Cach river in See also:Bokhara, but the word is probably of Tatar origin. Opaline silica is frequently deposited from hot siliceous springs, often in cauliflower-like masses, and is known as geyserite.

This occurs in Iceland, New See also:

Zealand and the Yellowstone See also:National See also:Park. The fiorite from the hot springs of See also:Santa Fiora, in See also:Tuscany, is opaline silica, with a rather pearly lustre. A variety containing an exceptionally small proportion of water, obtained from the Yellowstone Park, was named pealite, after the chemist A. C. See also:Peale. The siliceous deposits from springs, often due to organic agencies, are known generally as " siliceous See also:sinter " or, if very loose in texture, as " siliceous See also:tuff." Opaline silica forms the material of many organic structures, like the frustules of diatoms and the tests of radiolarians, which may accumulate as deposits of See also:tripoli, and be used for polishing purposes. (F. W.

End of Article: OPAL

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