Understanding Stone Codes

American Gem Trade Association (AGTA) Enhancement Codes and Designations

Gemstones are often enhanced to maximize their natural beauty. The American Gem Trade Association (AGTA) defines “enhancement” as “. . . any traditional process other than cutting and polishing that improves the appearance (color/clarity/phenomena), durability or availability of a gemstone.”

To facilitate transparency in gemstone buying, the AGTA has created a list of enhancement codes. The symbols below will help you understand what treatment(s) a stone has undergone so you can make informed purchases.


Any traditional process other than cutting and polishing that improves the appearance (color/clarity/phenomena), durability, or availability of a gemstone.

Natural Stones

The “N” symbol appears on the chart only for natural stones which are not currently known to be enhanced; however, the “N” symbol can also be used for other natural gemstones in the event that a gemstone has received no enhancement and the seller will provide a guarantee that there has been none.

Symbols for Specific Forms of Enhancement


“ASBL” is the tag code for products made of multiple layers or combinations of manufactured and/or natural material fused, bonded, or otherwise joined together to increase stability and/or imitate the appearance of a natural gemstone, create a unique design or generate unusual color combinations.


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


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.


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


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


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 illuminated 10X magnification H and F should be used.

Heating and Pressure

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


“IMIT” is the tag code used for a manufactured product fabricated in such materials as glass, ceramic, or plastic designed to imitate or resemble the appearance, but not duplicate the characteristic properties of a natural gemstone. “IMIT” is also the tag code for a simulant, which is defined as man-made single crystal product that is used to simulate the appearance, but not duplicate the characteristic properties of the natural gemstone it imitates.


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


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

Oiling/Resin Infusion

The filling of surface-breaking fissures with colorless oil, wax, resin, or other colorless substances, except glass or plastic, to improve the gemstone’s appearance.


The tag code may be used to describe “synthetic” materials that have essentially the same optical, physical and chemical properties as a naturally occurring counterpart.


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.


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


The impregnation of a colorless wax, paraffin, and oil in porous opaque or translucent gemstones to improve appearance.

Some of the above treatments are easier to detect than others, so a reputable gemstone supplier is crucial to ethical business. Just as enhancement codes should be available to you, treated gemstones should always be disclosed to consumers.

Education and full disclosure of all gemstone treatments protects you from legal trouble and keeps your reputation sparkling.

For more answers about all things gemstones, call our Gemstones Team at 1-800-877-7777 Gemstones: Option 6 BenchJeweler Articles

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