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Opalus, Opal

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240                 NATURAL HISTORY OF GEMS.
the play of colours depends become choked up with dust and grease through, wear ; although it is said this property may be revived by the hazardous operation of roasting the stone by a gradual heat. The Hungarian Opals exhibiting a uniform milkiness of surface, more or less iridescent, have from their greater density the advantage of resisting the effects of wear longer than any other sort ; hence their superior value. But infinitely greater is the beauty of the Mexican when recent, presenting an unmixed globide of green fire like the glowworm's lamp, or a ball of phos­phorus moistened with oil. Nevertheless of so porous a nature is this kind, that it becomes colourless-if wetted; and, however carefully preserved, changes to an opaque brown after a brief existence as a jewel, and consequently it has no value in the gem-market. Barbot warns us against the dealer's artifice of macerating inferior stones in oil (to be detected by the tongue) which forces out their colours in a transitory perfection.
Pliny justly observes (xxxvii 21), "the Opal differs at once very much and very little from the Beryl," referring, it would appear, to the glassy texture of its substance, common to both, yet with the grand distinction of its iridescence. It came next to the Emerald in value : and naturally so, being almost the only one of the really pre­cious stones of which the full perfection could be developed, by the simple process of polishing known to the Romans. For this stone is invariably cut en cabochon; and owing to its extreme softness, the polishing is a matter of time and care alone. This is the sole instance where Pliny has quoted the selling price of a precious stone : the famous Opal of Nonius, then still in existence, as large as a hazel­nut (the greatest magnitude known up to that time to be attained in the class), valued at 20,000/. of our money (vicies II. S.). Nonius was proscribed by M. Antony
Opalus, Opal Page of 384 Opalus, Opal
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