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.