Various chemical salts are used in staining chalcedony and agates. The green is produced by soaking either in a nickel salt solution or in a solution of chromic acid, and then heating. Or the stones may be immersed from a week to two months in a saturated solution of potassium dichromate, after which they are transferred to a closed container and exposed to the fumes of ammonium carbonate for two weeks. After drying, a gentle heat will produce the green color. For an apple green, a solution of nickel nitrate is used, and the ammonium carbonate treatment is omitted.

Lemon yellow colored stones are obtained by gently warming well dried specimens in hydrochloric acid for about ten to fourteen days in a regulated oven. The translucent, pale blue colored stones sold as “chalcedony” are stained with an ammonia compound, and its subsequent evaporation is the cause of the rapid fading. The dark blues, sold as blue agate, are obtained by soaking
first in a solution of ferric oxide and then in a solution of potassium ferrocyanide. A lukewarm solution is used, and one to two weeks are allowed for soaking. After washing, the agate is left for some days in a saturated solution of ferrous sulphate, re-washed, and then slowly dried.

The application of heat alone will accentuate the color of many agates and carnelians, but these are sometimes first impregnated with a solution of iron vitriol. The stones may be soaked in this liquid from two to four weeks, according to their thickness. If they are more than ten millimeters thick, success is doubtful. Drying and repetition is then resorted to, the drying being effected by gently heating from two to ten days in a closed receptacle. Both heating and cooling must be performed gradually, otherwise the stones fracture. Most carnelians and agates marketed have been so altered, including many Indian carnelians.

Rich brown tints, resembling some garnets, may be produced by soaking pale carnelians in brown sugar for some time and then carefully igniting. A necessary precaution at all times is first to remove all traces of oil and other matter. For this reason, the material about to be stained is generally bathed in caustic soda, dried, and then soaked in warm, dilute nitric acid for two or three days. The acid is gradually brought to boiling point, and continued repetition may be necessary to remove all iron and other stains.
The Chinese very rarely alter the stones which they work, and it can be taken as a general rule that Chinese jade and other stones which they work and carve are presented in their natural colors. Idar-Oberstein in Germany is the center of the staining and coloring of stones in Europe, and its products are to be found in every country.