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PRECIOUS STONES.

and blue tints. Sometimes, too, the patches of colour are of moderate and uniform size; sometimes they are large and irregular. The precious opal is, moreover, sometimes so milky as to be almost opaque; sometimes, as in many Queensland and Honduras specimens, it is nearly as transparent as glass. An intermediate condition, provided the fiery play of colours be well developed, is most highly prized; the best opals from Hungary and many of those from Mexico and from New South Wales are of this kind.

The opal consists essentially of silica, but it differs from quartz—that is, rock crystal—in two important particulars: it is vitreous, not crystalline, and it contains combined water. The precious or noble opal usually contains from 9 to 12 parts of water in 100; but it may be dried so as to lose for a time a small part of this moisture without injury to its beauty; in fact, the whole of the water present is not essential to the mineral. The specific gravity of opal is lower than that of quartz or rock crystal—about 2·2. Its hardness is about 6, or even as low as 5·5. Its fragility and softness, and its liability to injury from oily or greasy matter, render the opal unfit for a ring-stone, but it may be used to advantage, in pendants, bracelets, and ornaments for the head. A foolish but prevalent notion that the opal carries bad luck with it is of quite modern origin, but lowers the commercial value of the stone. Moreover, the great diversity in the quality of this gem renders it impossible to assign definite prices to opals of definite weights, though it may be said that a fine opal of 1 carat is worth about £2.

Besides the more usual form of precious opal, we have the Mexican fire opal, showing, along with a slight cloudiness, a rich orange-red hue like that of a glowing fire; and the harlequin opal, in which the brilliant patches of colour are small, angular, variously tinted, and uniformly distributed. The common or non-prismatic opal is found of many hues, rose-coloured, green, milky, agatoid, and with dendritic markings. Hydrophane is a variety which becomes