Gemstone ratings are as follows:
AAA = exceptional stone with excellent color, great cut resulting in lots of light getting through and the top rating for precious stones such as sapphires, emeralds, and rubies.
AA = top quality stone, great color, little if any visible inclusions and usually the best rating for semi-precious stones like citrine, amethyst and garnet.
A = good quality stone that has very few inclusions, good color, transparent and has a nice cut that lets light through to sparkle.
B = ok but not top color, still transparent stone with light inclusions.
C = usually not consistent color throughout the stone, inclusions are visible to the naked eye and stone is not as transparent so it doesn't sparkle much.
D = low value stone with major visible inclusions and usually rather dull as they are less translucent.
Jewelry is usually rated according to the stones it is made with/of.
To see what other enhancement codes mean, see our AGTA guide:
American Gem Trade Association Enhancement Codes
Answer contributed by Dale "Cougar" Armstrong