Two major grading systems are in fairly widespread use: the AAA-A system and the A-D system (also called the Tahitian system). These are the most accepted systems, and considered standard by nearly all reputable pearl dealers and pearl distributors.
Even these systems, however, can become misleading if a seller uses terms from the grading system (such as "AAA"), but uses them to describe a different quality pearl than that which the system is generally understood to be describing. Or a seller could use a term not in the grading system (such as "AAAA") to make it appear that the pearl is beyond even the highest standard quality -- when in reality, that seller's "AAAA" pearls are actually equivalent to the more-common "AAA" grade, and his "AAA" pearls might only be equivalent to the commonly used "AA." Use of "AAAA" or "AAA+" is considered bad taste, or even dishonesty, by most reputable pearl dealers and pearl distributors.
For reasons such as these, it's extremely important when purchasing pearls only, to be absolutely certain of the meaning of any descriptive terms used by the seller. If possible, ask to see a written description of each grading term, so that you know exactly what the grade implies. Reputable jewelers will be happy to comply with such a request. Only in this way will you be able to determine if the price the seller is asking is reasonable.
The AAA-A System
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.
Obviously, these grading categories are quite broad and leave room for interpretation and individual judgment. Also note that in multi-pearl pieces such as strands, necklaces, bracelets, etc., all of the individual pearls may not absolutely meet the indicated grade level. For example, a strand referred to as "AAA" must have most of its pearls as AAA pearls. However, a few pearls could have slightly lower luster or a tiny bit more surface defects. This is because matching is also a primary consideration in multi-pearl jewelry, sometimes even overriding a very strict grading of each individual pearl.
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."
A: The highest-quality pearl, with very high luster with only minor imperfections over less than 10% of its surface.
Pearls below D grade are considered not acceptable for use in jewelry.