Color: Red, reddish-brown, brown, green, yellow, orange, pink, purplish-red, white, colorless, black (Garnet occurs in every color except for blue)
Hardness: 6½ - 8½
Mineral Class: Garnet
Treatment: There are no treatments commonly used to enhance garnet.

Color: Light to dark purple
Hardness: 7
Mineral Class: Quartz
Treatment: Almost all amethyst is heated to bring out its best color. Heat-treated amethyst is very stable and requires no special handling.

Color: Light blue, blue, blue-green
Hardness: 7½ - 8
Mineral Class: Beryl
Treatment: Virtually all aquamarine is heated to minimize the greenish component of its color, making it appear a purer blue. Heat-treated aquamarines are generally stable, but their color may fade if exposed to prolonged periods of high heat or direct sunlight.

Color: Colorless, yellow, orange, brown, black. Rarely red, blue, green, pink, and purple.
Hardness: 10
Mineral Class: Diamond
Treatment: Diamond enhancements are specific treatments, performed on natural diamonds that are designed to improve their characteristics and value. There include clarity treatments such as laser drilling to remove inclusions and application of sealants to fill cracks. Color treatments to improve a white diamond’s color grade, or treatments to give fancy color to a white or off-color diamond.

Color: Emerald-green to dark green
Hardness: 7½ - 8
Mineral Class: Beryl
Treatment: Almost all emeralds are routinely enhanced to improve their appearance. Both natural and manmade fillers are commonly introduced into the fissures in emerald to reduce their visibility. Often referred to as oiling or infilling, similar enhancements have been done on emeralds for thousands of years. These types of enhancements are not considered stable, as fillers will come out over time or when exposed to high heat.

Color: White, cream, yellow, pink, peach, black, brown, gray, green, light purple, light blue
Hardness: 2½ - 4½
Mineral Class: Organic
Treatment: Most of the lighter shades of cultured pearls are mildly bleached to even out their color. Cultured pearls are sometimes tumbled to enhance roundness. They may be dyed to produce overtones of pink or strong body colors like teal, magenta or gold. Occasionally cultured pearls are irradiated to create strong iridescence or dark hues like black or gray.

Color: Bright red, brownish-red, purplish-red, dark red
Hardness: 9
Mineral Class: Corundum
Treatment: In the jewelry industry, it is assumed that rubies have been treated unless otherwise specified. Heat-treated rubies are very stable.

Color: Yellow, to yellow-green, olive-green, to brownish
Hardness: 6½ - 7
Mineral Class: Olivine
Treatment: There are no treatments known that could enhance the quality of peridot.

Color: Blue, yellow, green, white, colorless, pink, orange, brown, and purple
Hardness: 9
Mineral Class: Corundum
Treatment: Almost all sapphire on the market today has been heat treated to improve its appearance. Heat-enhanced sapphire is very stable.

Color: Colorless, white, yellow, orange, red, purple, blue, green, gray, brown, and black
Hardness: 5½ - 6½
Mineral Class: Opal
Treatment: There are various treatments used to enhance the beauty of opals. The most common treatments darken the body color, making the play of color slightly more noticeable. Some opals are coated with oil, wax or plastic to improve their appearance. All of these treatments only affect a thin outer layer of the gem’s surface and, therefore, are not considered stable.

Color: Pale yellow to brown
Hardness: 9
Mineral Class: Quartz
Treatment: Almost all citrine on the market today has been heat treated to improve its appearance. The color of citrine, whether treated or not, may fade if exposed to heat or sunlight for prolonged periods.

Color: Light to medium blue
Hardness: 7½
Mineral Class: Zircon
Treatment: Almost all Zircon gems are artificially colored by heat-treatment. Many of its gem colors are rarely found naturally in such color. An interesting and strange habit exhibited in only few zircons is that their color darkens and their luster dulls upon prolonged exposure to sunlight. This effect can be reversed by giving the stones a second heat-treatment.