Natural gemstones have not had anything done to them that changes their color, stability or durability. They may only be polished or cut to show off their beauty. Natural stones often have imperfections in them, and can be quite expensive.
Many gemstones are enhanced, which may improve or change the color, provide stability or increase durability. Stability means that the stone won't change color under normal conditions. Durable stones won't scratch or break when worn. The four main methods of enhancement are heat treatment, irradiation, chemical treatment and assembly. While some enhancements are done with an intent to deceive, many gemstones would not be usable in jewelry if they weren't treated in some way.
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. 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. Irradiation is not always permanent; some irradiated stones revert to their natural colors when exposed to extreme heat or light.
Heat treatment and irradiation are generally undetectable.
Chemical treatment of gemstones includes bleaching, dyeing and staining. It also includes the uses of oils, waxes, resins or plastics to stabilize or change the appearance of a gemstone. Chemical treatments are often called impregnation, because the chemicals usually penetrate the surface of the stone. Porous stones, like turquoise, are frequently sealed with wax or resin to keep the color from fading. Oils, waxes and plastics are used on many stones to hide small scratches and surface flaws. Coral, ivory and pearls may be bleached. Nearly all gemstones can be dyed or stained. Some chemical treatments are
