Gemstone treatment is any enhancement, by man, to a gem material other than cutting or polishing. Below is a list of gemstones, the method of known treatment and its frequency of use.

Alexandrite - NoneAmazonite - Usually, the use of wax, paraffin or oil via impregnation; occasionally, use of plastic or hardening agents.
Amber - Occasionally, heated for sun spangles or color enhancement; rarely, dyed or surface treated.
Amethyst - Occasionally, heated to lighten color and to remove 'smoky' components.
Ametrine - None
Ammolite - Commonly treated with colorless hardening substances, for stability, via impregnation.
Andalusite - None
Aquamarine - Usually heated to remove yellow components (green) to leave the blue.
Beryl - Blue-Always irradiated blue from colorless; Pink-Commonly heated to remove the yellow component; Yellow/Green-None; Red-None; Yellow-Usually produced by irradiation.
Chalcedony - Agate-Usually dyed; (Black) Onyx-Always dyed; Banded-Usually dyed; Blue-Commonly dyed; Green-Usually dyed; Carnelian-Usually heated, occasionally dyed; Jasper-Occasionally dyed; Chrysoprase-None.
Chrysoberyl - Cat's Eye-Occasionally irradiated; Yellow or Brown or Green - None.
Citrine - Usually produced by heating various types of quartz.
Coral - Black-None; White-Commonly bleached; Pink-Commonly impregnated with colorless wax; Orange-Commonly stabilized with plastic; Gold- Usually bleached from black; Red-Occasionally dyed.
Diamond - Occasionally to commonly laser drilled; Rarely coated; Occasionally by filling surface cavities or fractures with a hardened substance; Occasionally irradiated and/or heated. Now a recent procedure of taking a brown diamond and making it white (HTHP for high temperature high pressure).
Diopside - None.
Emerald - Usually the penetration of colorless oil, wax, unhardened resins into voids; Commonly the filling of surface cavities or fractures with a hardened colorless substance; Occasionally dyed with colored substance.
Garnet - None
Hematite - None
Iolite - None
Ivory & Bone - Commonly bleached; Occasionally dyed; Occasionally impregnated with colorless wax paraffin; Commonly impregnated with colorless hardened substances.
Jade - Commonly impregnated with colorless wax; Commonly bleached and polymer impregnated; Occasionally dyed.
Kunzite - Commonly heated and/or irradiated.
Lapis Lazuli - Commonly impregnated with colorless wax or oil; Commonly dyed.
Malachite - Occasionally coated with a colorless wax; Rarely impregnated with plastic and/or other hardened agents.
Moonstone - None
Opal - Rarely impregnated with colorless oil, wax and resins; Rarely impregnated with colorless plastic; Commonly sugar solution infilling in an acid bath; Occasionally infusion of unhardened essentially colorless substances into voids; Usually impregnated with colorless resins and hardeners.
Pearl - Usually bleached; Rarely dyed in natural pearls but usually dyed in cultured pearls; Occasionally irradiated; Commonly chemical treatment with heat.
Peridot - Rarely penetration of colorless oil, wax, unhardened resins into voids; Rarely filling of surface fractures with a colorless hardened substance.
Rhodochrosite - Rarely dyed.
Rhodonite - None.
Ruby - Usually heated; Rarely dyed; Commonly intentional filling of surface cavities and fractures with a foreign material, including glass; Commonly as a by-product of heat treatment the presence of solidified borax or similar colorless substances; Less rare now is diffusion of color or asterism.
Sapphire - Usually heated; Occasionally diffusion of color or asterism; Occasionally irradiation to provided temporary intense yellow or orange color. Diffusion sometimes and particularily in bulk.
Serpentine - Rarely dyed.
Spinel - None
Spodumene - Rarely irradiated green.
Sugelite - None
Tanzanite - Usually heated.
Topaz - Blue-Usually irradiated and often heated; Yellow/Orange-Occasionally irradiated; Pink/Red-Usually heated; Brown-None; Green-Usually irradiated.
Tourmaline - Chrome Vanadium-None; Cat's Eye-None; Yellow/Orange-Rarely heated or irradiated; Blue/Green-Commonly heated, Occasionally the penetration of colorless oil or unhardened resins into voids, Occasionally filling of surface cavities or fractures with a colorless hardened substance; Pink/Red/Purple-Occasionally heated, Commonly irradiated, Occasionally penetration of colorless oil or unhardened resins into voids, Occasionally filling of surface cavities or fractures with a hardened substance, Occasionally penetration of coloring agents into voids.
Turquoise - Commonly impregnated with plastic; Commonly impregnated with colorless oil or wax; Rarely dyed.
Zircon - Green/Brown-None; Yellow-Rarely heated; Blue/Red/Colorless- Always heated from the brownish crystals.Credit and thanks should be given to the AGTA -American Gem Trade Association for having the ethics to push the disclosure