O'PAL, a mineral which differs from quartz in containing from 5 to 13 per cent of water, its only other essential constituent being silica, although a little alumina, oxide of iron, etc., is often present. It is never found crystallized, mid does not exhibit a crystal line 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 and sometimes oriental opal. It is semi-transparent or trans lucent, usually of a bluish or yellowish white color, yellow by transmitted light, and exhibits a beautiful play of brilliant colors, owing to minute fissures which refract the light. It is much valued for setting in rings, brooches etc.. and is polished with a con vex surface, never cut into facets, both because of its brittleness, and because its play of colors is thus best exhibited. The ancients valued opals very highly. The Roman sena tor 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 ster ling. The imperial cabinet of Vienna contains the most celebrated opal now known to exist. It is 5 in. by 2:1- inches. The finest opals are almost all brought from Kaschan in Hungary, where they are found disseminated in a trachytic conglomerate.
They are mostly very small, bid 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, etc. When the colors are not equally dif fused, but in detached spots, jewelers call it harlequin °put. There is it dark or blackish variety, apparently tinged by oxide of iron, which occasionally exhibits very beautiful reflections, and is then much prized. Gh'asol (q.v.) and eacholong (q.v.) are varieties of opal. What lapidaries call prime d'opal is clay-porphyry, or other stone containing many small grains 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 colors. It is not is rare mineral, and is chiefly found in Semi-opal is Wood opal is a petrifaction, 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.