General Enhancement Categories :

1) N: Those natural stones which are not currently known to be enhanced by any methods, such as spinel, and therefore can safely be presumed untreated. The symbol "N" is used for these and may also be used in the case of other stones which are sometimes or often treated, which in the particular case are unenhanced. To use this symbol on sapphire, for example, which is generally heated means that the seller certifies that the particular stone was not heated and supplies a document such as an invoice or lab report so stating.
2) E: Those natural stones which are routinely enhanced by traditional methods, the particular stone given this designation, may or may not be enhanced. For example, since most emeralds are oiled an E would indicate such treatment, but would not cover non-traditional methods such as hardened plastic resins (like Opticon) which would require specific enhancement codes such as those listed below. Another example would be the use of E for aquamarine, which in most cases is heated prior to the sale of the rough to remove greenish tints. If the seller knows what specific treatment has been used, then a more specific code should be used.
3) Those gemstones for which definite information on standard treatments is known, or to which N and E codes do not apply, as they have been treated in non-traditional ways which must be specifically disclosed by the appropriate code and also in writing. For example, a ruby which has had fractures or cavities filled with glass would receive an I (infilling) code as described below. A Morganite, which is known to have been heated, would receive an H rather than the less specific, E.