Treated Gemstones

People alter almost all gems for use in jewelry by cutting and polishing them after they are found. There are also several other treatments that may be done to enhance their appearance.

Heat Treatment

It’s the exposure of a gem to high temperatures to alter its color and/or clarity. The heating of gemstones often happens in the countries where they were found. They do not need to be declared according to The World Jewellery Confederation CIBJO guidelines. Heat treatments of these gemstones is considered permanent under normal handling conditions. The most commonly encountered heat-treated gems include:
Amethyst – Heating can remove brownish inclusions in some amethysts or lighten the color of overly dark stones.
Aquamarine – Many aquamarines are naturally a blue–green color. Heating can remove the greenish color to produce a bluer appearance.
Citrine – Some forms of amethyst can be heated and turned into citrine.
Ruby – Heating can remove a purplish color and make the ruby more of a red color. Silk inclusions can also be removed to make a gem look lighter and more transparent.
Sapphire – Heating can intensify the blue color in sapphires and can also remove silk inclusions to make it appear more transparent.
Heating of rubies and sapphires can also cause recrystallization of the silk inclusions to make them more prominent. This allows the cabochon cut gemstone to have stronger asterism, which is a reflecting star effect.
Tanzanite – Heating tanzanite at low temperatures can remove the brownish color to produce a stronger purplish-blue color.
Topaz – Heating yellowish pink topaz sometimes removes the yellow color to show more of the pink color. Heating is also used to control the color of blue topaz. Colorless topaz that is irradiated followed by heating results in a blue color.
Tourmaline – Sometimes heat treating can cause a dark green color to become lighter and it can also affect the color of other tourmalines.
Zircon – Some reddish brown zircons are heated to produce other colors, including an intense blue.

High Pressure High Temperature Treatment (HPHT)

Heating of a diamond to high temperatures under high pressure can remove or change its color. Heating can remove or lessen a yellow or brown color so the gem becomes more colorless. Other types of diamonds may be changed from brown to yellow, orange-yellow, yellow-green, or to blue colors by this process. Only a qualified gem-testing laboratory can confirm the treatment. It is occasionally encountered in colorless diamonds, but more often in some colored diamonds. HPHT treatments are considered stable and permanent for normal jewelry wear and no special care needed.

Irradiation

It is the exposure of a gem to radiation to change its color. This is sometimes followed by a heat treatment to further modify the color. It is also known as a combination treatment. Blue topaz, diamond, and quartz tend to have very stable colors, if not exposed to high temperatures. There is no special care requirements for most irradiated gems. The most commonly encountered gems include:
Beryl – Some varieties of beryl can be irradiated to deepen or change the color. The color tends to fade upon exposure to bright light.
Diamond – Neutron and electron radiation are the most common forms and it is possible to produce green, blue, deep yellow, orange, pink, and red diamonds. It is often combined with a secondary step of heating to achieve additional colors. Naturally occurring strong colors (green, red, blue, purple, and yellow) are rare and very expensive. Strong colors in diamonds should be considered likely altered. Whether a colored diamond is a natural color or treated color requires examination by an experienced gem-testing laboratory.
Quartz – Some varieties of quartz can be irradiated to produce amethyst. In some quartz, heating after irradiation results in a green color.
Sapphires – There are naturally occurring orange sapphires, but in some stones, a bright orange color is produced from a natural pale yellow color. The color in these is not stable and fades upon exposure to bright light.
Topaz – Colors of natural topaz are rarely vivid. Colorless topaz can be subjected to radiation to change its color. Along with heat treatment, strong blue colors can be achieved. Irradiation is frequently encountered in topaz.

Impregnation

The surface of a porous gemstone is soaked with a polymer, resin, wax, or plastic to make it stronger and to improve its appearance. The most commonly encountered gemstones are opaque and they include turquoise, lapis, jadeite, nephrite, amazonite, rhodochrosite, and tiger’s-eye. Due to the melting point of these materials, it can be damaged by heat. A jeweler’s torch should not be used because it can damage the stone. The stone would need to be removed and reset when the piece requires this type of work. It is frequently seen and in most cases a qualified gemologist can identify the treatment.

Bleaching

It’s a chemical used to alter or reduce the color of a porous gem. Jadeite jade, pearls, chalcedony, and tiger’s eye quartz may be bleached to lighten their color. It is frequently encountered in pearls and jadeite.

Dyeing

It is adding a colored dye into porous or fractured gem to change its color. Pearls, turquoise, lapis, nephrite jade, chalcedony, quartz, emerald, and ruby may be dyed. A qualified gemologist can detect dyed gems in most cases. They are frequently encountered for colored pearls and occasionally for most gems. When it is known that gem has been dyed, be careful to keep them from chemicals, such as acetone or alcohol, which could dissolve the dyes. Also, keep them away from long periods of sunlight, which could cause the color to fade.