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.