According to Sir David Brewster the colours of the opal may be referred to the presence of multitudes of microscopic pores arranged in parallel lines, the differences of tint arising from differences in the magnitude of these cavities. Μ. Descloizeaux, a very high authority on the optical properties of minerals, has also connected the iridescence of the opal with a regular system of internal cavities, but at the same time seems disposed to think that the colours bear some relation to the presence of a small quantity of organic matter in the mineral. Mr Crookes refers the colours to delicate striae or minute fissures lying in different planes within the opal. The whole subject has been exhaustively studied by H. Behrens, who explains the colour as due not to reflexion from the walls of cavities but to the interposition of very thin lamellae possessing a different index of refraction from that of the matrix in which they are embedded. He has detected in the opal a variety of microscopic enclosures, including flecks of hydrophane and delicate laminae of ferric oxide. On the whole, therefore, it seems likely that the iridescence of the opal is simply a case of the well-known colours of thin plates. A ray of light is reflected from the anterior surface of an attenuated film; another ray, having traversed the lamina, is reflected from the posterior surface; and the meeting of these rays differing in phase gives rise to interference phenomena.

The noble opal, in consequence of its brilliant iridescence, is valued as an ornamental stone. It is displayed to best advantage when cut en cabochon or with a convex surface. Pliny relates that the rich Roman senator Nonius was proscribed by Mark Antony for the sake of a magnificent opalus which he possessed, a stone as large as a hazel nut, and valued at 20,000 sesterces.

The precious opal occurs in veins in trachytic rocks, chiefly in Hungary. The best-known localities are Czerwenitza and Kaschau, but the most brilliantly - coloured stones are said to come from Kremnitz and Dubnik. It is found also in Honduras, principally near Gracias a Dios, where it occurs in like manner, forming veins in volcanic rocks. The American opal is generally less milky and less fiery than the Hungarian. Of late years some very brilliant opal has been sent into the market from Queensland. This is found as thin films spread over the walls of fissures in ironstone nodules, occurring in the neighbourhood of the Baracorra river. It is generally too thin to be cut en cabochon, but is largely used for camei, the iridescent layers showing to great advantage against the ironstone matrix, which when polished assumes a dark-brown colour. The so-called “black opals,” which have lately become popular, consist of this matrix penetrated in all directions by veins and spots of opal, forming a mixture sometimes known as “root of opal.” Certain stones sold as black opals have evidently been modified in colour by staining or by heat.

See Brewster, “On the Cause of the Colours in Precious Opal,” Edin. New Phil. Journ., xxxviii., 1845, p. 385; Descloizeaux, Manuel de Mineralogie, vol. i., 1862, p. 22; Crookes, “On the Spectral Phenomena of Opals,” Quart. Joum. Science, vi., 1869, p. 481; and especially Behrens, “Mikroskopische Untersuchungen über die Opale,” Sitzungsber. d. k. k. Akad. d. 11 ιssensclιaften, 11 ιen: math.-naturwiss. Classe, lxvi., 1871, p. 519. For a mineralogical description of the varieties of opal see Mineralogy, vol. xvι. p. 390.