Gem treatment symbols:
ASBL Assembled from pieces of the stated material(s) along with binders, substrates, caps, or carriers.
F Cracks & crevices filled with a fairly durable material.
HP Heated under pressure.
I (same as S).
IMIT Imitation – a manmade, or (usually treated) natural, material designed to resemble the stated stone but not equivalent to it chemically and physically.
L Lasered, possibly with chemical treatment.
MM A manmade material having no natural equivalent. (Such materials are often not marked with this symbol if the very name of the material designates a material that is generally known to the trade to be manmade).
N Natural. This symbol is used either to emphasize that a material that the reader may imagine to be treated, is in this case untreated; or to designate that a stone that has a gemologically inaccurate colloquial name is in fact an untreated stone although not the stone the name suggests, e.g., undyed mountain “jade”.
O Oil, resin, or wax colorless infusion, usually in the context of a transparent stone penetrated by tiny voids that would otherwise be conspicuous, e.g. emeralds.
R Irradiated and possibly heated.
S Stabilized (impregnated with a resin usually suitable for subsequent cutting and polishing).
SYN Synthetic (laboratory duplicate of the natural stone, having its same chemistry and crystallography.)
U Chemicals diffused into surface under high temperature.
W Colorlessly waxed or oiled, usually in the context of a nontransparent stone.