Porous stones are the easiest to treat, for they will naturally absorb any coloring matter in the form of a solution much more readily than others which have a hard texture. Such stones would not be visibly porous and their surfaces might appear to be hard and smooth. Those of a crypto-crystalline structure seem to take an artificial stain the best, for instance, chalcedony and turquoise. Staining by the introduction of coloring matter almost always involves heating the specimen as well, so it will be seen that a controlled heat is a necessary factor in this process. Experience in judging the time and the amount of heat required is necessary; gradual application is essential, otherwise the stone will crack with the sudden and unequal expansion, and flaws will develop to such an extent that the material will be spoiled. With experience, the facility with which a rough stone will take a stain and the time required for the process can be approximately judged, and with some qualities of chalcedony, those which are most suitable for staining gain a higher price in the rough than others which are not so suitable for this purpose.
Chalcedony, a type of quartz which comprises several well known semi-precious stones, is most frequently used to produce an artificially colored but real stone. All colors are introduced, blues, greens, reds, pinks, and yellows, the blues and pinks usually being the most unstable. The translucent bluish material is sold as “chalcedony,” and the pink stones as “rosaline.” Both fade fairly quickly and unevenly, and a bead necklace of such stones will show differences of shade after wearing for a short time. The translucent greens, often of a very attractive shade, are more stable; they are marketed as “chrysoprase” or “green carnelian,” both misnomers, for there is a natural chrysoprase of a green color (always different from the artificial green), and green carnelian is a contradiction of terms. These stones are stained after cutting, but “chalcedony” and “rosaline” are stained in the rough before cutting.