The alteration by various artificial means of the natural color of stones is a recognized practice, but it is confined to a certain number of varieties, the chief of which is quartz. Colors are made more attractive, or a different color altogether may be introduced. In some instances, this alteration is permanent; in others, the color fades in course of time and with exposure to light. Blue is a particularly fugitive shade if artificially introduced.
In general, the addition of a color is only resorted to with stones of a porous nature. Some varieties are altered by heating alone, but the amount of heat necessary is a matter of experience and varies with individual stones. We have already noted the effect of heat on gem stones, and here we shall deal with improvement in color and quality from a commercial point of view by the use of this agent.
Such treatment of stones by a variety of processes is not a modern idea, for many writers, including Pliny, describe to us the different methods that were used many hundreds of years ago. Some of these methods still hold good; others have been improved, yet the majority of gem stones still come to us in their natural colors, just as they are found in the earth. One of the chief attractions of a gem stone, indeed, is the quality of its being perfectly natural, and there is a suspicion about anything of value that has been tampered with, or that has been altered in some way so that its commercial value has been increased. None of the precious stones, as far as is generally known, is subjected to alteration of color for commercial purposes, and the porous varieties of quartz, or chalcedony, constitute the main bulk of stones which have been improved or altered by artificial means. In general, these stones are among the least valuable of jewel stones (although by no means the least interesting), and poor material is often made more attractive by introducing a bright color or intensifying existing colors. Synthetic stones, of course, may be produced in any color, so the need for their alteration does not arise.