Opal

Opal, a mineral which differs from quartz in containing generally 3 to 10—in some cases only 1, in others as much as 21—per cent. of water, its only other essential constituent being silica, although a little alumina, oxide of iron, &c. is often present. The water is readily driven off on the application of heat, and some opals contain so small a proportion of water that they might be described simply as jelliform quartz. Not infrequently minute scales or plates of tridymite (a crystallised variety of silica) are present in opal. The latter is never found crystallised, and does not exhibit a crystalline structure like quartz. It has a conchoidal fracture, and is very easily broken. There are many varieties, which pass into one another, so that their precise limits cannot be defined, from which has arisen no little confusion of names. The finest kind is called Precious Opal or Noble Opal, and sometimes Oriental Opal. It is semi-transparent or translucent, usually of a bluish or yellowish white colour, yellow by transmitted light, and exhibits a beautiful play of brilliant colours, owing to minute fissures which refract the light. It is much valued for setting in rings, brooches, &c., and is polished with a convex surface, never cut into facets, both because of its brittleness and because its play of colours is thus best exhibited. The ancients valued opals very highly. The Roman senator Nonius preferred exile to giving up an opal to Mark Antony. This opal was still to be seen in the days of Pliny, who ascribes to it a value equal to more than £100,000 sterling. The imperial cabinet of Vienna contains the most celebrated opal now known to exist. It is 5 inches by 2½ inches. The finest opals are almost all brought from Cerwenitza, between Eperies and Kaschau, in Hungary, where they are found disseminated as alteration-products in trachyte tuff. They are mostly very small, but even a very small opal, if really beautiful, is worth four or five pounds; and the price increases very rapidly with increase of size. Precious opal is found also in Saxony, in South America, &c. When the colours are not equally diffused, but in detached spots, jewellers call it Harlequin Opal. There is a dark or blackish variety, apparently tinged by oxide of iron, which occasionally exhibits very beautiful reflections, and is then much prized. Girasol (q.v.) and Cacholong (q.v.) are varieties of opal. What lapidaries call Prime d'Opal is porphyrite or other igneous rock, containing many small amygdules of opal. It is cut into slabs, and made into boxes and other ornamental articles; the stone which contains the opals being often artificially blackened by boiling in oil, and afterwards exposing to a moderate heat. —Common Opal is semi-transparent, white, yellow, green, red, or brown, and does not exhibit any play of colours. It is not a rare mineral, and is chiefly found in veins and cavities or diffused (as an alteration-product) through the mass of various igneous rocks. Semi-opal is more opaque. Wood Opal is a petrification, and exhibits the form and structure of wood, the place of which has been taken by the siliceous mineral. Hyalite and Menilite are varieties of opal.