Many gemstones are treated in some fashion, for a variety of reasons. Many are treated to enhance or change their colour or clarity. Others are fragile and have to be treated to increase their durability. There are some stones that are routinely treated in order to emulate a different kind of stone. A reputable dealer will disclose any treated gems if known, as will any reputable jeweller. Failure to disclose such details may result in civil and/or administrative actions by buyers and gem associations. This is a listing of some of the most common treatments.
ASBL - Assembled
DBL - Doublet
TPL - Triple
B - Bleaching
C - Coating
D - Dyeing
E - Enhanced
F - Filling
G - Gamma/Electron Irradiation
R - Neutron Irradiation (regulated by the NRC)
H - Heating
IMIT - Imitation
L - Lasering
MM - Man-made
N - Natural
This symbol is reserved for stones that have not been enhanced or treated in any way. Thankfully, there is some natural beauty left in the world, and I try to go this route whenever possible.
I - Infilling
O - Oiling/Resin Infusion
S - Stabilization(Bonding)
SYN or CR - Synthetic/Created
U - Diffusion
Using certain chemicals alongside high temperatures can produce colours in some stones and also an effect called asterism (stars). Star sapphires or star rubies are good examples though they do occur naturally as well.
W - Waxing
The surface of a gemstone is covered in wax to improve the durability and appearance. It is not a permanent treatment, but it is easy enough to reapply the wax. Jadeite, turquoise, lapis lazuli and rhodochrosite are some examples of gems that are commonly waxed.