OPAL, an amorphous or non-crystalline mineral consisting of hydrated sihca, occasionally displaying a beautiful play of colour, whence its value as a gem-stone. It is named from Lat. opaltis, Gr. bwaWLOv, with which may be compared Sansk. upala, a precious stone. Opal commonly occurs in nodular or stalactitic masses, in the cavities of volcanic rocks, having been deposited in a gelatinous or colloidal condition. It is inferior to quartz in hardness (H. 5-5 to 6-5) and in density (S. G. 1-9 to 2-3), whilst it differs also by its solubihty in caustic alkahs. The proportion of 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 anomaly being due to tension set up during consohdation. The mineral when pure is transparent and colourless, as well seen in the variety which, from its vitreous appearance, was called by A. G. Werner hyalite (Gr. i;aXos, glass), or popularly " Miiller's glass," a name said to have been taken from its discoverer. This pellucid opahne sUica occurs as an incrustation in small globules, and is by no means a common mineral, being chiefly found at certain localities in Bohemia, Mexico and Colorado, U.S.A. (Cripple Creek).
The beautiful variety known as " noble " or " precious opal " owes its value to the brilhant flashes of colour which it displays by reflected Ught. The 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 foreign matter, or of opaline sihca, having a different index of refraction from that of the matrix. The origin of the colours in opal has been studied by Sir D. Brewster, Sir W. Crookes, Lord Rayleigh and H. Behrens. In the variety known to jewellers as " harlequin opal," the rainbow-like tints are flashed forth from small angular surfaces, forming a kind of polychromatic mosaic, whilst in other varieties the colours are disposed in broad bands or irregular patches of comparatively large area. By moving the stone, a brilliant succession of fiery flashes may sometimes be obtained. The opal is usually cut with a convex surface, and, being a soft stone, should be protected from friction hkely to produce abrasion; nor should it be exposed to sudden alternations of temperature. The loss of water, sometimes effected by heat, greatly impairs the colour, though moderate warmth may improve it. According to Phny the opal ranked next in value to the emerald, and he relates that the rich Roman senator Nonius was exiled by Mark Antony for sake of his magnificent opal, as large as a hazel nut. The opal, on account of its unique characters, has been the subject of remarkable superstition, and even in modern times has often been regarded as an unlucky stone, but in recent years it has regained popular favour and is now when fine, among the most highly valued gem-stones.
Precious opal is a mineral of very limited distribution. Though ancient writers state that it was brought from India, and fine stones are still called in trade " Oriental opal," its occurrence is not known in the East. The finest opals seem to have been always obtained from Hungary, where the mineral occurs, associated with much common opal, in nests in an altered andesitic rock. The fine opals occur only at the Dubnyik mine, near the village of V'orosvagas (Czerwenitza). The workings have been carried on for centuries in the mountains near Eperjes, and some remarkable stones from this locality are preserved in the Imperial Natural History Museum in Vienna, including an uncut specimen weighing about 3000 carats. Precious opal is found also in Honduras, especially in trachyte near Gracias a Dios; and in Mexico, where it occurs in a porphyritic rock at Esperanza in the state of Queretaro. A remarkable kind of opal, of yellow or hyacinth-red colour, occurs in trachytic porphyry at Zimapan in Hidalgo, Mexico, and is known as " 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 Australia. In Queensland it is found lining cracks in nodules of brown ironstone in the Desert Sandstone, a rock of Upper Cretaceous age, and is distributed over a wide area near the Barcoo 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 work or is carved into cameos which show to much advantage against the dark-brown matrix. The matrix penetrated by veins and spots of opal, and perhaps heightened in colour artificially, has been called " black opal "; but true black opal occurs in New South Wales. I'he " 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 White Cliffs, Yungnulgra county, where it is found in nodules and seams in a siliceous rock of the Upper Cretaceous series. It is notable that the opal sometimes replaces shells and even reptilian bones, whilst curious pscudomorphs, known as " pineapple opal," show the opal in the form of aggregated crystals, perhaps of 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 Ireland, the west of Scotland, the Faroe Isles and Iceland. When of milky-white colour it is known as " milk opal "; when of resinous and waxy appearance as "resin opal"; if banded it is called "agate opal"; a green variety is termed "prase opal "; a dark red, ferruginous variety " jaspar opal "; whilst " rose opal " is a beautiful pink mineral, coloured with organic matter, found at Quincy, near Mehun-sur-Yevre, in France. A brown or grey concretionary opal from Tertiary shales at Menilmontant, near Paris, is known as menilite or " hver opal." A dull opaque form of opal, with a fracture imperfectly conchoidal, is called "semi-opal"; whilst the opal which not infrequently forms the minerahzing substance of fossil 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 America as " magic stone." Cacholong is another kind of porous opal with a lustre rather like that of mother-of-pearl, said to have been named from the Cach river in 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 Zealand and the Yellowstone National Park. The fiorite from the hot springs of Santa Fiora, in 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. Peale. The siliceous deposits from springs, often due to organic agencies, are knovyn generally as " siliceous sinter " or, if very loose in texture, as " siliceous 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 tripoli, and be used for polishing purposes. (F. W. R.*)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)