The A-D Pearl Grading System

A
Highest-quality pearl, very high lustre and only minor imperfections over less than 10% of its surface.

B
High or medium lustre. Surface may have some visible imperfections over no more than 30% of its surface area.

C
Medium lustre with surface defects over not more than 60% of the surface area.

D
May have many slight defects over 60% of surface or deep defects over no more than 60% of surface.

These grading systems are used to determine the quality of the pearl.  However, there are other factors that contribute to the value and pricing of pearls.