Though not commonly known to the public, many gemstones are treated in some way to enhance their color and clarity. These treatments are common and widely accepted in the industry, and normally do not de-value the stone.
Most retail jewelers do not divulge this information to their buying public and sometimes they themselves are not aware of it. If they are a reputable and ethical establishment, they will be honest if queried on the subject. Don't expect this information to be offered without inquiring first.
The most common treatments used on natural stones that I have come across are listed and explained here.
Heat treatment is the exposure of a gem to high temperatures to alter its color and/or clarity. This is the most common method used to color enhance gems. The most common gems that use heat treatment are: amber, aquamarine, amethyst, citrine, ruby, sapphire, tanzanite, topaz, tourmaline and zircon. Heat treatment is considered durable and permanent under normal handling conditions. The heat may render the gems slightly more brittle than usual, so care
must be taken not to damage pointed faceted corners and edges.
Impregnation stabilizes porous gemstones
using a colorless bonding agent, such as polymer, wax or plastic, to add durability and improve appearance. The most common of these gemstones include: turquoise, lapis lazuli, jadeite, nephrite,
amazonite, rhodochrosite and serpentine. Many impregnations are often “skin deep” and due to the melting point of
plastic and wax, can be susceptible to heat damage. Plastic
impregnations are considered durable in gem materials such as turquoise
as long as they are not subjected to heat or chemicals. Care must be taken not to subject gemstones with wax or plastic
impregnations to heat, such as that encountered by a jeweler’s torch,
since these will likely melt the material.
Fracture or Cavity Filling is filling surface-reaching fractures or cavities with a glass, resin,
wax or oil to conceal their visibility and to improve the apparent
clarity of gem materials, appearance, stability, or in extreme cases—to
add to a slight amount of weight to a gem. The filling materials vary
from being solids (a glass) to liquids (oils), and in most cases, they
are colorless (colored filler materials could be classified as dyes). The most common gems that use this treatment are: diamond, ruby, emeralds, quartz, aquamarine, topaz, tourmaline and other transparent gems. Durability? Much depends on the filler. Glasses tend to be harder
and therefore more durable than resins, oils or waxes. Changes in air
pressure, proximity to heat, or by exposure to chemicals can all affect
the appearance of filled gems by potentially altering or removing the
Diffusion uses chemicals in conjunction with high temperatures to produce color and/or asterism, or star like, producing
inclusions as in the star sapphire.
Bleaching is using a chemical to alter or rid a porous gem of all of it's color. Some gemstones are bleached and then dyed, a form of
“combination treatment.” Jadeite jade, pearls, coral, chalcedony and tiger’s eye quartz may be bleached to lighten their color.
Oiling and/or Resin Infusion is filling the surface of breaking fissures in gems with colorless oil, wax, resin
or other colorless substances, except glass or plastic to improve the
Waxing is the impregnation
of colorless wax, paraffin and oil in porous, opaque or translucent gemstones
to improve appearance.
There may be more ways in which gemstones are treated to enhance their color and durability. These are the more common ones I have come upon as a gemstone buyer. If possible, try to purchase untreated gemstones. Know that because they are more rare, they will also be a higher price
than treated gemstones.
(Information for this article has come from 2 sources: from the blog Unicorn's Garden and the GIA website)