Since ancient times, gemstones have been treated in a variety of ways to increase their beauty. An example is the bleaching of bone or ivory to improve the color. There exist many techniques to enhance the appearance of gemstones and in fact nearly all gemstones on the market today have been treated in some way, as they have in the past. In addition, new techniques have been developed in recent years (for example, laser treatment and irradiation) and new ones are being developed all the time. This only becomes a problem if consumers are led to believe that their gemstones have not been treated, and are worth more than they really are for this reason. A partial list of the common treatments of gemstones is given below.
| Agate | Dyed |
| Aquamarine | Heated to improve color |
| Black Onyx | Dyed |
| Carnelian Agate | Dyed |
| Citrine | Heated to produce color |
| Coral, White | Bleached white coral |
| Coral, "Gold" | Bleached black coral |
| Diamond, colorless | Laser drilled and/or filled to improve appearance |
| Emerald | Oiled with colorless oil to improve appearance |
| Ivory & Bone | Bleached to whiten |
| Jadeite | Impregnated with wax |
| Lapis Lazuli | Dyed; Impregnated with wax or oil |
| Pearl, natural | Bleached to improve color and appearance |
| Pearl, cultured | Bleached to whiten, dyed for colors |
| Ruby | Heated to improve color |
| Sapphire | Heated to optimize color |
| Serpentine | Dyed |
| Sodalite | Dyed |
| Tanzanite | Heated to produce color |
| Topaz, Blue | Irradiated or heated to produce color |
| Topaz (Yellow/Orange) | Irradiated to intensify color |
| Topaz, Pink | Heated to produce color |
| Tourmaline (Green/Blue) | Heated to improve color |
| Turquoise | Impregnated with oil or wax to enhance color |
| Turquoise | Stabilized with plastic to improve color & durability |
| Zircon (Blue, Red & Colorless) | Heated to produce color |