Opal, a mineral composed principally of silicic acid combined with from 5 to 13 per cent, of water. The finest varieties have the most delicately resplendent play of iridescent colors, with a peculiar vitreous, sometimes resinous and pearly lustre. It is softer than quartz, the hardness ranging from 5.5 to 6.5; specific gravity 1.9 to 2.3. The varieties pass into one another, so that it is difficult to classify those which form the transitions. Dana gives the following: 1. Precious opal, spoken of by Pliny as presenting various refulgent tints in succession, now one hue and now another. The specimens are rarely larger than a hazel nut; there is one in the Vienna museum the size of a man's fist, weighing 17 oz., but having many fissures. 2. Fire opal, of a hyacinth red to honey yellow, with fire-like reflections. 3. Girasol, bluish white, translucent, with reddish reflections of bright light. 4. Common opal, including among other kinds resin opal, semi-opal, hydrophane (which becomes translucent or transparent in water, whence its name, though this is a common quality of opal), and forcherite (orange yellow, colored by orpiment, from Reittelfeld in Upper Styria). 5. Cacho-long (Kasclwlong, Perlmutter), opaque, bluish white and reddish, adhering to the tongue, containing a little alumina.

G. Opal agate. 7. Menilite (Leieropal). 8. Jasper opal (Eisenopal), containing yellow oxide of iron, and having the color of yellow jasper. 9. Wood opal, consisting of wood petrified by opal. 10. Hyalite, clear as glass and colorless, constituting globular concretions, also crusts with uniform surface, passing into translucent. 11. Fiorite, silicious sinter, occurring in tufa, in the vicinity of Santa Fiora, Italy, and at the solfatara near Naples, in globular and stalactitic concretions, resembling hyalite, but pearly in lustre. Thomson mentions a similar incrustation formed from the hot waters of the Sasso lagoons. Another variety of fiorite is michael-ite, from the island of St. Michael, one of the Azores, where it is found in snow-white incrustations, capillary in structure and pearly in lustre; also geyserite, from the Iceland geysers, having porous, stalactitic, cauliflowerlike forms. 12. Float stone (Schwimmstein), in light, tuberose, spongy masses, floating on water, sometimes having a flint-like nucleus. 13. Tripolite, an earthy variety of opal, formed from the silicious shells of diatoms and other microscopic species, discovered by Ehrenberg, and occurring in deposits often many miles in area, containing several sub-varieties, some uncompacted and some moderately hard. - The precious opal was known to the ancients, and ranked among the most valuable gems.

Pliny describes it under the name of opalus, and in the Orphic hymns it is spoken of as the, and again as iraidtpuQ in allusion to the delicacy of its complexion, like that of a child, g. Fine specimens of precious opal are still valued as gems. Opal in some of its varieties is not rare. It occurs in veins in porphyry, sometimes associated with galena and blende, and again in vesicular cavities in amygdaloidal rocks, sometimes in limestone and clay slate, and even as the material replacing the organic matter of fossils. It is obtained in various parts of the world. Mines have been worked for it in Hungary, in the-county of Saros, for several centuries, and the precious opal extracted and taken away by Grecian and Turkish merchants has, it is said, found its way by the Indies to Holland under the name of oriental opal. The same variety is also brought from Honduras and Nicaragua. The fire opal is found in the Faroe islands, Guatemala, Zimapan in Mexico, Washington co., Ga., and various other places. Humboldt brought from Mexico the first specimens of it seen in Europe. It is too rare to be employed like the precious opal in jewelry. The latter is used for rings, necklaces, and other ornaments, usually in oval or lens form, and to best advantage in a black setting.

The edges of the stone on account of its softness are easily injured, and it must therefore be used with care. Very fine jewels are sometimes protected by a thin plate of quartz crystal. Changeable red and green colors are the most highly valued. Some opals are rated at very high prices, as one at £4,000 from Hungary in the great exhibition of 1851, which weighed 526½ carats. Two belonging to the crown jewels of France cost 75,000 francs. Specimens imported into the United States are valued by dealers at $4 to $10 a carat. Some of the finest are from Gracias a Dios, Honduras. Wood opal is named from its peculiar ligneous structure, and occurs in the form of trees in trappean rocks. Its localities are Hungary, France, Iceland, Greenland, Maryland, Pennsylvania, and Colorado.