Agate: can be dyed
Amethyst: Not always treated. If treated, heat treatment can lighten the color. Can also be produced by irradiating citrine to turn it from yellow to purple.
Aquamarine: heat treatment can change green material to blue.
Carnelian: can occasionally be dyed; heat treatment can improve color.
Citrine: Not always treated. If treated, can be produced by heat treating amethyst to change color from purple to yellow.
Diamond: Colorless diamonds are generally untreated. However, they can be laser drilled, the impurities bleached, and then fracture filled to improve clarity. Light yellow and brown diamonds can be irradiated to change color to green, brown, blue, yellow, orange, and more rarely, pinks and reds.
Diopside: no known treatments.
Emerald: Traditionally, for hundreds of years, almost all emeralds have been treated by soaking in oil to improve appearance. The oil penetrates the surface reaching fractures and makes them appear less noticeable. It can also help to increase color saturation. Today, while still routine, this practice has taken on a more advanced level. Instead of oil, emeralds are commonly treated with a high-grade epoxy resin, such as Opticon (registered), to improve their appearance and increase durability. The 5.76 carat emerald featured on the "Green with Envy" page has been treated in this more advanced fashion. These treatments are not permanent and can all be reversed. Because it is well known that all emeralds are treated, this has virtually no effect on value.
Garnet: no known treatments.
Iolite: no known treatments.
Jade (both jadeite and nephrite): can be bleached, dyed, polymer impregnated, or heat treated- or a combination of these. The trade typically refers to three types of jadeite: 'A' jade, 'B' jade, and 'C' jade.
'C' jade is the most common type of jade and accounts for most jade found in jewelry. It is usually bleached, dyed, and polymer impregnated.
'B' jade is a natural color jade that has been bleached to remove any dark, unattractive discolored areas. It is also often polymer impregnated (which increases the durability of this already tough gem material).
'A' Jade is completely untreated in every way and is both very rare and expensive.
Kunzite: can be (but very uncommonly) irradiated
Kunzite Note: Unrelated to enhancement--Exposure to intense sunlight over a very long period can cause the gradual fading of this pink gem.
Lapis: commonly dyed and polymer impregnated or oiled to improve color and luster.
Malachite: very uncommonly impregnated with a polymer/epoxy resin.
Moonstone: no known treatments.
Opal: I use untreated opals and occasionally opal doublets. Opal doublets are an assembled stone that combines, via epoxy, a thin layer of opal backed by either chalcedony or boulder rock.
Pearl: Routinely bleached to remove discoloration. White pearls are dyed to achieve a pink overtone (rose overtone). While some pearls are dyed to change color completely, most dark colored Chinese pearls are irradiated to change their color. The pastel colors of Chinese freshwater pearls are supposedly untreated and therefore of natural color.
Pearl note: Mabe pearl: Mabe pearls are cultured 'blister pearls.' In the culturing process, instead of inserting a bead nucleus 'free-floating' into a mollusk or oyster to get a round-shaped pearl, the nucleus is attached to the inner lining of the shell. The mollusk then begins to 'cover' this irritant with calcium carbonate, composed of calcite and aragonite, the two minerals that make up the pearly nacre of a pearl. When the mollusk is harvested, the pearl that has formed like a blister on the inner shell is cut out and backed with mother of pearl or shell.
Pearl note: Keshi pearls: Keshi pearls are entirely untreated. After being removed from the mollusks, they are washed only with a gentle soap and rinsed with water.
Peridot: no known treatments.
Rock Crystal Quartz: no known treatments.
Ruby: can be heat treated, diffusion-treated (I do not sell), and occasionally oiled and dyed (I do not sell).
Blue sapphire--can be heat-treated or diffusion treated (I do not sell)
Smoky Quartz: usually natural; very black material is usually irradiated.
Tanzanite: routinely heat-treated.
Blue: irradiated followed by heat treatment.
Pinks & Reds: commonly irradiated.
Greens: often heat-treated.
Turquoise: can be polymer impregnated
Note: Turquoise is often reconstituted, and this can be hard to detect. Reconstituted turquoise involves grinding up pieces of turquoise, pressing them into solid pieces with an adhesive binder such as an epoxy, and then recutting.
*Important note to gemstone enhancements: While the following list describes many of the changes to various gemstones, gemstone enhancement is not an exact science. Results of treatments are not always predictable. For example, two gemstones of the same color from the same geographic location and deposit, when heat-treated, may turn completely different colors.