GRADING GUIDE for CULTURED FRESHWATER PEARLS
The chart below, from the website of jewelry supplier Fire Mountain Gems (www.firemountaingems.com), explains how they grade cultured freshwater pearls. Fire Mountain Gems says they use the same pearl grades the mainland Chinese pearl traders use. Here is the Chinese pearl grading standard from Fire Mountain Gems:
A Grade: Extremely well shaped, white, high luster, very minor surface blemishes.
B Grade: Well shaped, white to light cream, high luster, minor surface blemishes.
C Grade: Irregular shape, off-white, medium luster (may be non-uniform), surface is irregular.
D Grade: Same as C grade, but one or more of the characteristics are excessively bad (such as areas of no luster or wrinkled surfaces).
F Grade: For promotional purposes only. Not suitable for jewelry. Most of this grade is ground up into pearl powder (used in cosmetics).
"Some pearls fall between grades and may be assigned mixed grades (A-B grade). In addition, a pearl with A grade luster and C grade surface may be given a compromise B grade. It may be best to avoid mixed-quality pearls." (firemountaingems.com)