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Some American Gem Trade Association® (AGTA) Codes and definitons are listed below. At the end is the source page link for further information.

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

N 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 stones in the event that a stone has received no enhancement and the seller will provide a guarantee that there has been none.

E The “E” symbol indicates that a gemstone is routinely enhanced.

SYMBOLS FOR SPECIFIC FORM OR ENHANCEMENT

ASBL=Assembled: The process of producing a single stone from two or more separate pieces.

B=Bleaching: The use of chemicals or other agents to lighten or remove a gemstone’s color.

C= 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.

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

F=Filling: As a by-product of heat enhancement, the presence of solidified borax or similar colorless substances which are visible under properly illuminated 10X magnification.

G=Gamma/Electron irradiation: The use of gamma and/or electron bombardment to alter a gemstone’s color, may be followed by a heating process.

H=Heating: The use of heat to effect desired alteration of color, clarity, and /or phenomena. (Residue of foreign substances in open fractures is not visible under properly illuminated 10X magnification.

I*=Infilling: The intentional filling of surface breaking cavities or fractures usually with glass, plastic, option with hardeners and/or other hardened foreign substances to improve durability, appearance and/or weight.

L=Lasering: The use of a laser and chemicals to reach and alter inclusion in diamonds.

O=Oiling/Resin Infusion: The intentional filling of surface breaking cavities of a colorless oil, wax, natural resin, or unhardened man-made material into fissured transparent/translucent gemstones to improve appearance (i.e., oil, man-made resin, cedar wood oil, Canada balsam, paraffin, etc.).

R=Irradiation: The use of neutron, requiring an environmental safety release from the Nuclear Regulatory Commission (NRC), with the combination of any other bombardment and/or heat treatment to alter a gemstone’s color.

S*=Bonding: The use of a colorless bonding agent (commonly plastic) within a porous gemstone to give it durability and improve appearance.

SYN=Synthetic: Man made. Note: A synthetic gemstone created in a laboratory is nearly identical to the natural gem in physical appearance, chemical composition and optical properties. Cubic Zirconia (CZ) does not have a corresponding gemstone that occurs in nature.

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

W*=Waxing/Oiling: The impregnation of a colorless wax, paraffin and oil in porous opaque gemstones to improve appearance.

*Asterisk near code denotes new classification
*Source: AGTA 1997/1998 Source Directory