While an international grading system does not exist for pearls, each producer, wholesaler and retailer sets their own standards. Most use the traditional A (lowest) through AAA (highest) grading scale.
The industry does recognize seven different factors to assign value: nacre quality, luster, size, shape, color, surface, and matching.
NACRE: Nacre is the outer coating of material that makes a pearl look like a pearl. The quality of nacre involves how thick and dense it is. Thickness is contingent on how long the mollusk is in water before it is harvested. Minimum thickness is 0.4 millimeters and anything less will eventually peel. Pearls that are harvested in the coldest season of the year deposit nacre at a slower rate because mollusks’ metabolism slows. The result is a dense, compact nacre and higher luster.
Mollusks are left in water for at least ten months, although at this stage the nacre is thin with lower than average luster. Most mollusks are in water for 18 or 24 months.
LUSTER: Luster is the most important value factor, which is determined by the amount and the quality of light reflected from the surface and just under the surface of a pearl. Fine luster is a sharp, shiny reflective surface, whereas low luster is dull or chalky reflections. The finest luster exhibits a prismatic effect, due to light passing through layers of nacre and reflecting back through the surface - also known as “orient”.
SIZE: The larger the pearl, the greater the value when all other factors are equal.
SHAPE: Most mollusks are nucleated with a perfectly round bead implant, resulting in round (the most prized) or nearly round pearls. Often, other shapes like drop, oval and baroque occur and can be quite valuable - especially baroque pearls. Baroque pearls are fancy shapes that are asymmetrical and free-form.
COLOR: The color of pearl is dictated by the lip color of the pearl, as well as the species of mollusk tissue that is grafted around a nucleus. Pearl color is comprised of two components: body color and overtone. Body color is the primary color caused by pigment versus light interference. Overtone is the secondary color that appears due to nacre layers interfering with light and splitting the components.
SURFACE: Just like our fingerprints, the surface of each pearl is unique to their individual growth characteristics. Bumps, dimples, scratches, dull patches and spots which may be present on the surface of a pearl are known as “blemishes”. The fewer blemishes (when all other factors are equal), the more valuable the pearl is.
MATCHING: Because no two pearls are identical, a lot of time can be involved in composing a matching pair of pearls in terms of of size, quality, color and shape. Even more so with a finely matched single strand of pearls. It requires time and a skilled technician to sort through thousands of pearls to identify the closest match. For the rarest of pearls, this could take years!