Luster, also termed as Briliance of a diamond indicates the brightness of the incident light, reflected back from within the diamond, as a result of the combined preciseness of the size and angles of its table, crown & pavilion facets.
At times, the rough diamond itself may inherently contain extremely miniscule but closely grouped bubble like structures or strands that may appear like a ‘cloud’ structure, which ultimately adversely affect light return of a diamond.

SGS Luster Grades Description
EX Excellent Luster
VG Very Good Luster
GD Good Luster
FR Fair Luster

Beside, some diamonds may have a little whiteness or cloudiness present in them. These are known as milky luster diamonds. Such diamonds are further classified into sub categories depending on the extent of the milky effect in them, using the below terminologies:

SGS Milky Grades Description
ML-1 Very Slightly Milky
ML-1.5 Slightly Milky
ML-2 Medium Milky
ML-2.5 Medium Heavy Milky
ML-3 Heavy Milky
ML-3.5 Very Heavy Milky
ML-4 Strong Milky