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PRECIOUS OPAL. 233 It does not appear that the opal was ever much used for engraving on, and very few engraved stones of this mineral are preserved in collections. The opal is fre quently minutely disseminated through the porphyry; and pieces of this kind, when cut and polished, are work ed for ornamental articles, as snuff-boxes, &c. Observations. 1. The pecular play of colour distinguishes this mine ral from all others. In all other characters it nearly agrees with Common Opal, differing principally in its higher degree of lustre and transparency. 2. This is one of the few minerals whose name has re mained unaltered from the earliest times; but its origin or derivation is imperfectly known. Some derive it from the Greek word <4 or otto?, which signifies vision, because it Was supposed to possess the power of strengthening the e ye. The precious opal was the only kind known to the ancients. 3. The finer varieties are named oriental opal. Taver- mer, however, informs us, that no precious opal is found m the East, and that those which are sold as oriental are brought from Hungary. 4. Those varieties of precious opal that adhere to the tongue, are only translucent, and scarcely exhibit any of the play of colour which so remarkably distinguishes the common varieties; but when immersed in water, they become more transparent, and acquire a very beautiful play of colour. This property occurs also in some varie ties of common and semi-opal; and the older, and some modern mineralogists, considered these varieties as con- *tituting a particular species, to which was given va rious