Pearl Luster is defined by the sharpness and brightness of reflections as seen on the surface of a pearl. The Gemological Institute of America classifies pearl luster into five grades.

Pearl Luster
Excellent	Reflections are uniformly bright and sharp.
Very Good	Slightly less defined, fairly sharp, bright reflections.
Good	Reflections are not clear but still fairly bright.
Fair	Reflections are very fuzzy and weak.
Poor	Reflections are non-existent or extremely weak.