Gems - Opal
( Originally Published 1919 )
( White Opal, Black Opal, Fire-Opal)
THAT opal in early times excited keen admiration is evident from Pliny's enthusiastic description of these stones : " For in them you shall see the burning fire of the carbuncle, the glorious purple of the amethyst, the green sea of the emerald, all glittering together in an incredible mixture of light." During much of last century, owing to the foolish superstition that ill-luck dogs the footsteps of the wearer, the species lay under a cloud, which has even now not quite dispersed, but exercises a prejudicial effect upon the fortunes of the stone. It has, however, recently attracted considerable attention owing to the discovery of the splendid black opals in Australia ; at one moment black with the darkness of night, at the next by a chance movement glowing with vivid crimson flame, such stones may justly be considered the most remarkable in modern jewellery. At the present day opal is divided by jewellers roughly into two main groups: ' white' (Plate XXVII, Fig. 6) and ' black' (Plate XXVII, Fig. 9), according as the tint is light or dark, fire-opal (Plate XXVII, Fig. z 0) standing in a separate category.
Opal differs from the rest of the principal gem-stones in being not a crystalline body, but a solidified jelly, and it depends for its attractiveness upon the characteristic play of colour, known, in consequence, as opalescence (cf. p. 39), which arises from a peculiarity in the structure. Opal is mainly silica, SiO2, in composition, but contains in addition an amount of water varying in precious opal from 6 to i 0 per cent. As the original jelly cooled, it became riddled throughout with cracks, which were afterwards generally filled with opal matter, containing a different amount of water, and therefore differing slightly in refractivity from the original substance. The structure not being quite homogeneous, each crack has the same action upon light as a soap-film, and gives rise to precisely similar phenomena ; the thinner and more uniform the cracks, the greater the splendour of the chromatic display, the particular tint depending upon the direction in which the stone is viewed. The cracks in certain opals were not filled up, and therefore contain air. Such stones appear opaque and devoid of opalescence until plunged into water ; they are consequently known as hydrophane, from vowp, water, and to make appear. Owing to the effect of total-reflection, light was stopped on the hither side of the cracks before they were filled with water, which is not far inferior to opal in refractivity; it is surprising how much water these stones will absorb.
Opal is colourless when pure, but is nearly always more or less milky and opaque, or tinted various dull shades by ferric oxide, magnesia or, alumina. The so-called black opal is generally a dark grey or blue, and very rarely quite black. That the coloration is not due to ordinary absorption, but to the action of cracks in the stone, is shown by the fact that the transmitted light is complementary to the reflected light ; the blue opal is, for instance, a yellow when held up so that light has passed through it. In many black opals the opalescent material occurs in far too tiny pieces to be cut separately, and the whole iron-stained matrix is cut and polished and sold under the name opal-matrix.' The reddish and orange-coloured stones known as fire-opal have pronounced colour and only slight milkiness ; they display the customary opalescence in certain directions. These stones are often faceted, but otherwise opals are cut en cabochon, either flat or steep—generally the former in brooches and pendants, and the latter in rings. Opal is somewhat soft, varying from 5 to 6 on Mohs's scale, and is therefore easily scratched. The specific gravity ranges from 2.10 to 2.20, and the refractive index from 1.444 to 1.464, the refraction, of course, being always single. It is unwise to immerse opals in liquids on account of their porosity.
The name opal comes to us through the Latin opallus, which was used for the same species as understood by the term at the present day, but the word has a far older origin, which has not been traced. The Romans also called the mineral paederos, the Greek form of Cupid, a name applied to all rosy stones. The name cacholong, for the bluish-white procelain variety, which is very porous and adheres to the tongue, is of Tartar origin ; the stone is highly valued in the East.
The oldest mines, which up to quite a recent date were the only extensive deposit of opal known, were at Cserwenitsa, near Kashau, in Hungary. From them in all probability emanated the opals known to the Romans. The opals from this locality were generally quite small, and large pieces were rare and commanded high prices. The Hungary mines, however, proved quite unable to compete with the rich fields at White Cliffs, New South Wales, in spite of the efforts that were made to depreciate and exclude from the market the new stones, and at the present time few of the opals on the market come from them. As so often happens, the White Cliffs deposit was discovered by accident. In 1889 a hunter, when tracking a wounded kangaroo, chanced to pick up an attractively coloured opal. The district is so waterless and forbidding that, but for such a chance, the opals might have long lain hidden. They occur in seams in deposits of Cretaceous Age in a variety of ways, filling cavities in rocks or sandstones, or cracks in wood, or replacing wood, saurian bones, and some spiky mineral, which may have been glauberite. In recent years, another rich deposit was discovered farther north, on both sides of the boundary between Queensland and New South Wales. The field is remarkable for the darkness of its opals, which are called ` black opal' in contradistinction to the lighter-coloured stones previously known. From Lightning Ridge in New South Wales come stones stained deep black which quite merit the designation black opal. The sandstone in which they are found is rich in iron, and this is no doubt responsible for the deepness of their tint. Mexico is noted for the fire-opal, which is found at Esperanza, Queretaro, and Zimapan ; but other kinds of opal also are found at these places.
The price of opal varies greatly, according to the intrinsic colour and the uniformity and brilliance of the opalescence. Common opal can be bought at as low a rate as Is. a carat, while black opal ranges from 10s. to £8 a carat ; but a good dark stone displaying a flaming opalescence commands a fancy figure, fine stones of this class being exceedingly rare. Fire-opal enjoys only a limited popularity now, though a few years ago it was in some demand ; the price runs from 2S. to 10S. a carat.