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Onyx, Nicolo

Onyx, Nicolo Page of 384 Onyx, Nicolo Text size:minus plus Restore normal size   Mail page  Print this page
232
NATURAL HISTORY OF GEMS.
wheels of various diameters, worked as the others, and supplied with diamond-dust and water. The Carnelian beads are finished by putting a number into a bag in which they are shaken together."—(' A Visit, December. 1832, to the Carnelian Mines, situated in the I'ajpeepta Hills, to the eastward of Broach. By Lieutenant G. Fulljames.')
Both the Onyx and the Sardonyx are now largely imi­tated by artificially preparing the German species, a stratified quartz, in quality little superior to common flint, by which treatment factitious stones are produced of much beauty, exhibiting well defined and even layers of black and white, or red and white, equally clear and vivid.* This is effected by boiling the stones continuously, for the space of three weeks, in a solution of honey in water, constantly replenished as it evaporates, and afterwards steeping them in sulphuric acid to bring out the black and white ; in nitric for the red and white colours.f The more porous strata absorb the honey first, and, secondly, the acid that carbonizes it—the more compact remaining unaffected by either. Such imi­tative gems are of very trifling value, being imported in vast quantities from Germany, where the secret was either discovered some years ago, or else, as others assert, intro­duced from Italy, where it had been practised from time immemorial. Pliny himself notes that all gems are bright­ened up by boiling in honey (mellis decoctu nitescunt), especially in the Corsican, noted for its acridity, although all other acids are detrimental to them (xxxvii. 74). There
* The true antique Sardonyx lias a peculiarity that serves, in addition to the superior beauty of its colours, to distinguish it infallibly from these German counterfeits. Its black substratum when viewed by trans­mitted light, turns to a rich chocolate-red highly translucent, whereas the same part of the artificially stained recent species continues as black and opaque as before.
t Another sort display shades of a beautiful sapphirine produced by a similar application of the prussiate of potash.
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