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

E - Enhanced Routinely: The "E" symbol indicates that the gemstone is normally enhanced. The type of enhancement is then indicated by the second symbol. Many gemstones are so routinely treated that it is extrememly rare to find them in a natural state.

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 elecron 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.

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

N - None, Natural:The "N" symbol is used for natural stones and those which have received no enhancement.

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

O - Oiling/Resin Infusion: The filling of surface breaking cavaties with a colorlesss oil, wax, natural resin, or unhardened man-made material into fissured transparent/transluscent gemstones to improve appearance.

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