The term chalcedony, derived from χαλκηδών the name of a town in Asia Minor, is usually confined to stones of a greyish tinge. Stones artificially coloured an emerald green have been cut and put upon the market as ‘emeraldine.’ Carnelian is a clear red chalcedony, and sard is somewhat similar, but brownish in tint. Chrysoprase is apple-green in colour, nickel oxide being supposed to be the agent. Prase (cf. [p. 240]), which is a dull leek-green in hue, may also in part be referred here; the name comes from πράσμον, a leek. Plasma, which may have the same derivation, is a brighter leek-green. Jasper is a chalcedony coloured blood-red by iron oxide, while bloodstone is a green chalcedony spotted with jasper; they are popular stones for signet rings. Flint, an opaque chalcedony, breaks with a sharp cutting edge, and was much in request with early man as a tool or a weapon; its property of giving sparks when struck with steel rendered it invaluable before the invention of matches. Hornstone is somewhat similar, but more brittle, while chert is a flinty rock.
Agate, named after the river Achates in Sicily, where it was found at the time of Theophrastus, has a peculiar banded structure, the bands being usually irregular in shape, following the configuration of the cavity in which it was formed. Moss-agate, or mocha-stone, contains moss-like inclusions of some fibrous mineral. Onyx is an agate with regular bands, the layers having sharply different colours; when black and white, it has, in days gone by, been employed for cameos. Sardonyx is similar in structure, but red and white in colour. Agate is used in delicate balances for supporting the steel knife-edges of the balance itself and of the panholders, and is largely employed—especially when artificially coloured—for umbrella handles and similar articles.
Chalcedony and agate are found the whole world over, but India, and particularly Brazil, are noted for their fine carnelians and agates.
CHAPTER XXX
OPAL
(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. 10) 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 10 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 ὕδωρ, water, and φαίνεσθαι, 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 6½ 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.