Many gemstones have to be altered in order to enhance their appearance. This is usually done to produce colors not usually found in nature. Enhancement techniques improve color, clarity, reduce porosity, stabilize color and enhance durability. Depending on the stone and the treatment, such alteration may be easy or impossible to detect. It is unethical and unlawful to sell any artificially enhanced gems without full disclosure of information about the treatment.
Examples of some of the methods frequently used are: heat treatment, irradiation, impregnation, and assembled stones.

HEAT TREATMENT
Many gems are heated under controlled conditions to improve color as in, (aquamarine, sapphire, ruby, and tourmaline), alter color (sapphire, amethyst to citrine, topaz, zircon), or improve clarity (sapphire, ruby). Since natural heating also occurs in volcanic areas, the artificial effects are indistinguishable. In most cases, the results of heat treatment are permanent.

IRRADIATION
Colorless topaz, yellowish diamonds are irradiated in large quantities and sometimes heated to produce various shades of blue, and a variety of colors. Other stones, such as tourmaline, are sometimes irradiated to produce new colors. In many cases, the effects of irradiation are somewhat unstable and can be reversed by heating.

IMPREGNATION AND CHEMICAL TREATMENT
Turquoise being porous is sealed with wax, plastic resin to "stabilize" and improve the color. Such material cannot be seen. "Black onyx" ,agate has been impregnated with sugar, and then carbonized by acid. Yellowish diamonds are sometimes coated on the girdle or pavilion with a thin bluish film to improve color.

Jadeite is sometimes chemically "bleached" and impregnated to improve color, and this treatment can be difficult to detect.

In recent years, a new treatment for corundum has appeared, in which poorly colored corundum is heated in chemicals to deposit a very thin layer of enhanced color on the surface of the stone. These stones can be quite impressive, but when re-cut the coloration comes off. Such treatment is easily detected by immersing the stone in a liquid with a high refractive index; where the color appears to concentrate along facet edges. Diffusion-treated blue sapphires and Rubies are also known.

ASSEMBLED STONES
Thin seams of opal are often assembled with backing of opal or black onyx to produce a doublet, and a clear quartz top is added to produce a triplet. This makes the material useful while the dark backing enhances the play of color and quartz top makes it durable. Opal doublets and triplets must still be protected against heat and liquids. If the adhesive layer begins to break down, the stone's appearance is marred, and it is difficult, to repair the damage.