Reputable suppliers disclose known enhancements at the time of purchase. The results of heat treatment and irradiation often mimic what Mother Nature would have achieved had the stones been left in the ground a few more centuries. It is usually impossible to detect which gemstones have been enhanced by either of these methods. Chemical enhancements are generally used to change the surface characteristics of gemstones, and assembly is often used to protect fragile stones. Heat treatment is commonly used to improve color.
Color changes resulting from heating are permanent in most gemstones.
Amethyst, ruby, sapphire, topaz, tourmaline, and zircon all are routinely heat-treated. Most aquamarine now sold has been heated, to change it from its natural green to the blue that is currently popular. Irradiation is also used to change or deepen gemstone colors. Clear topaz is irradiated to produce blue topaz, and colorless tourmaline may be changed to any one of several colors. Some diamonds are irradiated to improve their color.