As with any other item that can appear in a wide variety of shapes, sizes, colours and conditions a standardised system of grading must be used when purchasing or selling cultured pearls. Only in this way can the pearl be described according to mutually agreed-upon and understood terms, so that both, buyer and seller can determine a fair price.
Unfortunately the pearl industry as a whole has not adopted a universally used standard grading system. Instead, the specific grading system used often depends upon the specific jeweller or pearl distributor.
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 jewellers 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 pearls 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 lustre and at least 95% of the surface will be free from any type of defect.
AA:The surface will have a very high lustre and at least 75% of the surface will be free from any type of defect.
A:This is the lowest jewellery-grade pearl with a lower lustre and/or more than 25% of the surface showing defects. In many cases, if the pearl is being mounted into a piece of jewellery, it can be mounted so that the defects are hidden -- thus providing a lovely jewellery piece at a lesser price.
Some reputable sellers may use intermediate grades for the pearls (Perlen) - only those that do not fall in a category but are between two - such as A+ and AA+. Obviously these grading categories are quite broad and leave room for interpretation and individual judgement. 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 lustre or a tiny bit more surface defects.This is because matching is also a primary consideration in multi-pearl jewellery, 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 lustre and only minor imperfections over less than 10% of its surface.
B:High or medium lustre. Surface may have some visible imperfections, but over no more than 30% of its area.
C:Medium lustre 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 lustre 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 jewellery. Both of the grading systems described above focus primarily on the lustre and surface quality of the pearl to determine its grade. But keep in mind that other factors also contribute to the quality 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 millimetres. 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.