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American Gem Trade Association Codes (AGTAC)

The list of symbols below describes various forms of gemstone enhancements as defined by the American Gem Trade Association. An enhancement is any treatment process other than cutting and polishing that improves the appearance (color/clarity/phenomena), durability or availability of a gemstone.


Natural: No modification (or currently has no known modification process).


Bleaching: The use of heat, light and/or other agents to lighten or remove a gemstone's color.


Coating: The use of such surface enhancements as lacquering enameling, inking, foiling or sputtering of films to improve appearance, provide color or add other special effects.


Dyeing: The introduction of coloring matter into a gemstone to give it new color, intensify present color or improve color uniformity.


Filling: The filling of surface-breaking cavities or fissures with colorless glass, plastic, solidified borax or similar substances. This process may improve durability and/or appearance, and/or add weight.


Heating: The use of heat to effect desired alteration of color, clarity and/or phenomena. If residue of foreign substances in open fissures is visible under properly iluminated 10x magnification, H F should be used.


Heating and Pressure: The use of heat and pressure combined to effect desired alterations of color, clarity and/or phenomena.


Impregnation: The impregnation of a porous gemstone with a colorless agent (usually plastic) to improve durability and appearance.


A simulant, which is defined as a man-made single crystal product that is used to simulate the appearance, but not duplicate the characteristic properties of the natural gemstone it imitates.


Lasering: The use of a laser and chemicals to reach and alter inclusions in gemstones, usually diamonds.


Oiling/Resin Infusion: The filing of surface-breaking fissures with colorless oil, wax, resin or other colorless substances, except glass or plastic, to improve the gemstone's appearance.


Irradiation: The use of neutrons, gamma rays or beta particles (high energy electrons) to alter a gemstone's color. The irradiation may be followed by a heating process.


Diffusion: The use of chemicals in conjunction with high temperatures to produce ARTIFICIAL color change and/or asterism-producing inclusions.


Waxing/Oiling: The impregnation of a colorless wax, parafin or oil in porous opaque or translucent gemstones to improve appearance.


Synthetic: Synthetic materials that have essentially the same optical, physical and chemical properties as a naturally occurring counterpart. In all cases, stones must be referred to as "synthetic ruby," "laboratory grown sapphire," or "Chatham-Created emerald," etc.
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