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It is possible, by various processes, to improve the natural color of some stones by artificial means. One of the simplest methods is by fire. Experience and care are necessary, how­ever, as the stone is liable to damage. White spots are removed from rubies by burning. The color can be more or less burned from the jargoon. Smoky rock crystal is clari­fied by carefully heating in a crucible with lime, sand, or charcoal. The usual method of burning is to heat the stone in a crucible with unslaked lime, or iron filings, or sand and iron filings. The pink tint of Brazilian topaz is the result of burning. The tint of Oriental carnelian is also produced by exposure to heat. The red color of carnelian is produced by steeping in a solution of copperas or ferrous sulphate and then subjecting the stone to heat.
Onyx is dyed by soaking it, after being thoroughly washed and dried, for a period of two to three weeks in a mixture of honey and water, or sugar and water, or in oil. It absorbs this to a certain extent, some more, some less, according to the porosity of the layers in the stone. It is then taken out, washed, and placed in another dish with sulphuric acid. This is covered and placed in hot ashes, with burning charcoal over the cover. The acid carbonizes the saccharine matter in the pores of the stone, and the carbon gives to it a black or dark-brown color. It is then washed again, dried in an oven, and laid for a day in oil to increase the brilliancy. The results of this process are not uniform, some stones being very slightly affected only.
Agate is colored blue by first soaking it in a solution of yellow prussiate of potash and then in one of a ferric salt,