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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
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