Fluorescence in diamonds is graded on the following basis:
Emission of visible light by a diamond when it is exposed to ultraviolet radiation is termed as Fluorecence. A very high fluorescence reduces the briliance of a diamond.

Fluorescence color:
Depending on the nature and type of bonds that other elements form with carbon atoms of a diamond crystal, fluorescence can be of different colors.
Fluorescence colors like white, blue, yellow, orange and green are having direct impact on the overall color and hence the pricing of a diamond.

SGS Fluorescence Grades Description
N None Fluorescence
FNT Faint Fluorescence
MED Medium Fluorescence
STG Strong Fluorescence
VST Very Strong Fluorescence