The following is from www.pearl-guide.com
This system grades pearls on a scale from AAA to A, with AAA being the highest grade. This grading scale is common to freshwater and akoya pearls only, but is accepted by many with South Sea and Tahitian pearls as well:
- AAA:The highest-quality pearl, virtually flawless. The surface will have a very high luster, and at least 95% of the surface will be free from any type of defect.
- AA: The surface will have a very high luster, and at least 75% of the surface will be free from any type of defect.
- A: This is the lowest jewelry-grade pearl, with a lower luster and/or more than 25% of the surface showing defects. In many cases, if the pearl is being mounted into a piece of jewelry, it can be mounted so that the defects are hidden -- thus providing a lovely jewelry piece at a lower price.