Scientific American 41, 1.7.1848
Lapidaries are accustomed to improve and change the colors of gems by exposing them to heat, and other chemical agents.
In India, yellow cornelians are put into an earthen pot, covered with dry goats' dung, and heated for twelve hours, by which they are changed into a fine red. Intead of goats dung, sand may be used.
Black rock crystal is rendered colorless by heat, if continued for some hours, otherwise it will be only yellow.
Bucquet made a chemical distinction between rock crystal and quartz; the latter cracking by heat, probably on account of containing water.
The amethyst by a moderate heat becomes colorless; but if the heat is violent, white ad shotten like an opal; it is more liable to crack than rock crystal.
Beryl is changed y a oderate heat to a light blue, if the heat is greater, it becomes like mother of pearl.
The emerald acquires the same peray lustre by heat.
The color of the chrysoberyl is not altered by heat.
Blue four spar s changed to red, and if the heat is strong, and often rendered colorless.
Agates absorb oil, either by being immersed or biled in it for a sufficient time, or evenduring the process of cutting them, and on boiling them in oil of vitriol, the parts which have absorbed the oil are rendered black, while the other parts retain their natural color, or even become whiter than they were before.
Agates and cornelians having carbonate of soda applied to them, and then applied to the heat of a furnace under a muffle, an opake white enamel is thus made to cover the stone which cannot easily be distinguished from a natural white flake, By this means are produced the cornelian beads brought: from India, which are ornamented with a ew work of a white color, penetrating to a small depth, an equally as hard as the stone itself.
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