Pearls are classified by origin, then graded by size, shape, nacre thickness, luster, color, surface clarity and matching. There is no universal grading system for judging pearls. Each jeweler or pearl distributor uses their own independent judgment for pearl interpretation or grading.

Furthermore, each pearl type is only graded in comparison to other pearls of its own type. For example, South Sea pearls grow in warmer waters for longer periods of time than Japanese Akoya pearls developing in colder waters for shorter periods of time. Therefore, South Sea pearls tend to have lower luster and more tiny blemishes than Akoya pearls. For that reason, South Sea pearls are graded relative to the best available quality in other pearls of its own type, not by comparison to Akoya pearls.

Two major grading systems are prevalent amongst most reputable pearl dealers: the AAA-A system and the A-D system (a.k.a. the Tahitian system). 

·      AAA pearls reveal a very uniform color distribution, highly spherical shape, strong nacre and orient (iridescence), as well as a very high, uniform luster. Although most natural or cultured pearls possess some natural blemishes, at least 95% of the AAA pearl surface is free from any visible defect.

• AA pearls differ by showing a slightly less even color. The surface has very thick nacre, high luster and orient. At least 75% of the surface will be free from any type of defect or spotting, preferably close to the drill hole if strung in a necklace. Matching in necklaces within both these categories should be very good.
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• A pearls show a lower luster and/or blemishes on more than 25% of the surface. Flaws may be visible to the naked eye on close inspection and shapes may be slightly out of round. Matching will not be as good in necklaces of this grade. In many such cases, the pearl is mounted onto a piece of jewelry such as pendants or earrings so that the defects are not visible -- thus providing a lovely jewelry piece at a lower price.

The Tahitian System is only used to grade Tahitian and South Sea pearls based on the Tahitian government standards. Other markets still utilize AAA-A grading system. Under the Tahitian system, a pearl’s quality is defined by thickness of nacre layer, luster and surface purity.

Tahitian pearls are famous for their high gloss luster. There are four grades of pearl luster: Very High (AAA), High (AA), Medium (A) and Soft (B).

Luster is the quality of the reflection of light on the pearl’s surface. High, or very high luster exists when the reflections are bright and sharp. But when the light reflections are diffuse or fuzzy, the pearl has a soft or dull. Pearls with dull luster are rejected.

Nacre thickness affects the durability of a Tahitian cultured pearl. Tahitian pearls have a great reputation for thick nacre due to the long period of time and care necessary for cultivating them. Official classification regulations require a Tahitian cultured pearl to have at least 80% of its nucleus' surface covered with successive layers of pearl material. All exported or commercialized Tahitian pearls are required to have a minimum nacre layer thickness of 0.8 MM. Pearls with thinner nacres are rejected.

Surface purity is measured by the presence or absence of slight imperfections on the pearl surface. Quality is defined by four sub-categories:

• A - The highest quality pearl: one or more visible imperfections on less than  10% of the surface, with remarkable luster.
• B - visible imperfections on less than 30% of the surface, with reasonable or regular luster.
• C-  visible imperfections on less than 60% of the surface, with medium luster.
• D- visible imperfections on more than 60% of the surface, with dull luster. The pearl may show many slight, but not deep defects, spread over 60% of its surface; or it may have 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. However, if the pearl surface is graded D due to the extent of defects, the pearl luster is deemed irrelevant.

Pearls below D grade are not acceptable for use in jewelry, but may be used for ornamental or pharmaceutical purposes.