Generally, the colors of natural stones remain a permanent quality, but there are some which fade in time although they have not been artificially treated. Blue zircons, yellow and blue topaz, amethyst, and rose quartz are among those that will fade in strong sunlight or in course of ordinary wear. White zircons sometimes change to a dirty gray or brownish color, which was no doubt their original color before they were heated. Opals also should not be put in the strong sunlight, otherwise they might develop faults and lose some of their color, for this stone contains by nature an appreciable percentage of water.