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, SiO_{2}, 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.