The A-D System (or Tahitian System)

This system grades pearls on a scale from A to D, with A being the highest grade. This is the system used in French Polynesia (based on a government standard there) to grade Tahitian pearls, and South Sea pearls only. It is therefore sometimes referred to as the "Tahitian system." While this system is standard in producing countries, other markets will still utilize AAA-A.
* A: The highest-quality pearl, with very high luster and only minor imperfections over less than 10% of its surface.
* B: High or medium luster. Surface may have some visible imperfections, but over no more than 30% of its area.
* C: Medium luster with surface defects over not more than 60% of the surface area.
* D: May have many slight defects, but no deep ones, spread over 60% of its surface; or deep defects over no more than 60% of its surface; or a combination of minor and deep defects over no more than 60% of its surface. In this grade of pearl, the luster is irrelevant. Even the most lustrous pearls will be graded D if their surface is blemished to this extent. Pearls below D grade are considered not acceptable for use in jewelry.

Nacre Is The Final Factor In Grading Pearls
Both of the grading systems described above focus primarily on the luster and surface quality of the pearl to determine its grade. But keep in mind that other factors also contribute to the quality and final grade of any pearl. One of the most important is the thickness of the nacre, which often determines how durable the pearl will be over time. The thicker the nacre, the stronger and longer-lasting the pearl (provided it is treated well, of course!) For Tahitian pearls, the government of French Polynesia has set a minimum nacre thickness of 0.8 millimeters. Any pearls with nacre of less than that thickness are not allowed to be sold. Keeping in mind that Tahitian pearls tend to be larger than many other pearls (such as akoyas), you can use this rule as a guideline when evaluating your own potential pearl purchases.