OPAL, a mineral, in chemical composition an amorphous hy drated silica, some forms of which are highly prized as gems. Many varieties of opal (Lat. opallus, Gr. 67r6,XXcov), are recognised but of these few have any value as gem stones. The most beau tiful is precious opal which displays a wonderful scintillating coloured brilliance, known as opalescence. In early times it excited the keenest admiration, witnessed by Pliny's enthusiastic descrip tion, "For in them you shall see the living fire of the ruby, the glorious purple of the amethyst, the green sea of the emerald, all glittering together in an incredible mixture of light." Orange or yellow stones which exhibit opalescence are called fire opals. Black opals are stones in which the background is extremely dark. Really black stones are extremely valuable and rather rare. Com mon opal is a term applied to the varieties which do not exhibit opalescence. Most of the nan-_es applied are self-explanatory, e.g., milk opal, resin opal, liver opal, agate opal, etc. Prase and jasper opals are green and red respectively. A curious, very porous variety, which can absorb surprising quantities of water is called hydrophane. It will adhere to the tongue. It is almost opaque when dry, but becomes practically transparent when saturated with water. Another porous variety is cachelong, which has a lustre like mother of pearl.
Opal is the silicious material of the tests of radiolaria and the frustules of diatoms. These may accumulate as deposits of tripoli or kieselguhr, used for polishing. Opal is often found as pseudo morphs after gypsum, glauberite, calcite and other minerals. Pseudomorphic aggregates are sometimes known as pineapple opal. Opal has been discovered in a fibrous form much like asbestos, from which it may be distinguished by its harsh touch.
Opal is widely distributed as nodules and stalac titic masses in the cavities of volcanic rocks, deposits from hot springs, etc. It is deposited as a gel or sol. Precious opal is found only in a few places. Cserwenitsa in Hungary was for long the only source ; probably all the ancient stones were found there. Since the discovery of the rich fields in Australia, the Hungarian mines have lost most of their importance. The discovery of opals at White Cliff, New South Wales, was followed in a few years by the field on the boundary between New South Wales and Queens land. Many "black opals" are found there, but truly black stones are found at Lightning Ridge. The black opal has been thus described :—"It combines the iridescence of the dewdrop with the colour of the rainbow, set in the blackness of night. It is a smoth ered mass of hidden fire." Precious opals have been found in Hosako, Japan, which are nearly colourless and transparent with a bluish tint, and show a change rather than a play of colours, from emerald green to apple red. Esperanza, Queretana and Tima
pan in Mexico are noted particularly for fire opals. A fine mass of precious opal was found recently in Nevada.
Pure opal, which is colourless, is rarely found. It is usually stained dull colours by ferric oxide, alumina, lime or magnesia. That the coloration is not ordinary absorption is shown by the fact that the transmitted light is complementary to the reflected. If for example a blue stone is held up to the light, it appears yellow. The opalescence is not due to any pigmentation, but to the fact that as the original gel dried and cooled it became riddled with cracks, which were subsequently filled up by another gel containing a different amount of water. The resulting hetero geneity of the opal gives it a varying refractive index, which affects the light in the same way as a soap bubble. The thinner and more uniform the cracks the greater the splendour of the colours, the shade depending on the direction in which it is viewed. In the variety known as harlequin opal the rainbow colours are flashed from little angular surfaces forming a mosaic.
Opals are usually cut en cabochon. If they are too thin for this, they are used for inlay work, and if scattered in small pieces throughout the matrix, they are sold as such, under the name "root of opal." Fire opal shows to best advantage if facetted. Opal is not placed among the most precious of stones because of its sof t ness, but Pliny placed it of ter the emerald. He relates that Mark Antony exiled a rich senator, Nonius, for the sake of an opal the size of a hazel nut. Many superstitions have centred round opal, and even in modern times it has been regarded as unlucky.
Opal contains from 3-13% of water, precious opal between 6 and
It is soluble in caustic alkalis, and when mixed with soda easily fusible. It is soft, 5.5-6 on Mohs' scale and is theref ore easily scratched. It is brittle and has a con choidal fracture. Opals are porous, and it is dangerous to immerse them in liquids. Normally opal is isotropic, but owing to internal strains it is sometimes doubly refracting. The refractive index ranges from 1.444-1.464. They are said to be more brilliant on warm days, but a high temperature, by withdrawing water, de stroys their value. Recently milk opal has been coloured by oils and pigments to imitate the rare blue and dark red stones. The colour is fixed with Canada balsam.
See A. Eppler, Die Schmuck- and Edelsteine (Stuttgart, 1912) ; G. F. Herbert Smith, Gemstones (London, 1926). (W. A. W.)