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Crystallus, Rock-crystal

Crystallus, Rock-crystal Page of 453 Crystallus, Rock-crystal Text size:minus plus Restore normal size   Mail page  Print this page
CRYSTALLUS.
179
to the time of steeping. Such false Rubies are still known to the French lapidaries according to Barbot by the name of Rubaces. But the substance of the Crystal is not really coloured ; it is merely the dye which, insinuating itself into the innumerable minute fissures produced by the sudden cooling, that apparently stains the entire mass ; and the great art in the process is so to regulate the heat that these fissures be not too apparent upon the surface of the pretended gem.
It is, however, probable that the ancients employed the more simple method now so much in use, and which produces most of the Carbuncles seen in the London shops, viz., after cutting the Crystal into shape, then to paint the back with the required colour, and so to set it with a backing. The fact that anciently gems were for the most part set in such a manner would greatly favour the execution of this fraud, to baffle which Pliny expressly notices that the better kinds of Jaspers and Chrysolithi were mounted à jour, " funda perspicua." Although the Roman jewel­lers made up the false Sardonyx of three layers by cementing together as many slices of different coloured stones, yet they do not seem to have been acquainted with Doublets, that favourite device with the modern trade, in which a thin slice of"the true gem, improved by cementing a paste of the proper colour under­neath, assumes the appearance of a first-class stone in its kind ; or where, the operation being reversed, the paste surface is backed by a facetted Crystal to supply the required lustre. The ancient frauds in the matter of coloured gems went no farther than the above-named staining of the Crystal, and the substi­tution of pastes for stones. To detect the latter trick, Pliny lays down (76) many rules, some fanciful enough, but amongst them one infallible test, that a splinter of Obsidian will scratch a paste but not a real gem.
To conclude the subject of False Gems, which naturally falls under the head of Crystal, so much employed, either in its natural form, or chemically combined, in their composition, we may insert some of the curious observations of Camillo Leonardo, written before 1502, as to the various forgeries practised by the jewellers of his own times. Many of these are extremely inge­nious, and doubtless the recipes had been preserved in the tra-
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