Understanding the 7 Factors of Cultured Pearl Grading
On the surface, a cultured pearl grading system wouldn’t seem difficult to create, as there are seven common factors everyone agrees are vital in establishing pearl quality: luster, surface quality, size, shape, color, nacre thickness, and (for strands or sets) matching. But try to quantify the degree to which any of these factors affects the quality and value of a pearl, and it very quickly snarls into a complicated issue.
ABOVE: multicolored Japanese Akoya pearls. Photo by Tammy Jones.
If we were only talking about the Japanese Akoya–round and white (more or less) pearls–a grading system might even now be on its way to being internationally accepted. In fact, GIA did have one largely geared toward Akoya pearls. But Akoyas are no longer the only game in town (and they’re no longer only produced in Japan). There are South Seas pearls, Tahitian pearls, and Chinese and other freshwater pearls, and each producer feels that their product should have a system that represents their merits. For South Seas, shape is important; with Tahitians it’s the colors and overtones; and for American freshwater, it’s their natural, undyed state. And how do we deal with mabé pearls, described as an “assembled product” by GIA?
7 Factors of Pearl Grading
So while there is no established pearl grading system, there are established pearl quality factors to consider when buying pearls. While these attributes contribute to the beauty and value of a pearl, not everyone agrees on their order of importance. Nor do they all agree that all factors come into play.
Luster is the amount and quality of light reflected from a pearl’s surface. It’s the most important factor in pearl quality, across the board. When it comes to luster, Akoyas are the ones to beat. The surface luster of an Akoya is very strong, due to the relatively colder waters that produce the nacre that reflects light. Some dealers consider Akoyas to have the finest luster of all cultured pearls.
It is not nacre thickness alone that gives a pearl higher luster, but the uniform way the platelets in the nacre are stacked. Platelet arrangement is also responsible for two other optical effects seen in pearls: overtone and orient. Both overtone and orient result from light diffraction at the edges of aragonite platelets that make up the nacre coating.
Where the aragonite platelets meet each other on the surface, they create “suture lines.” If these lines are curved and run deep, you get rainbow colors called orient. If the sutures are straight and run in parallel lines, you get the purplish-pink and green overtones of Tahitian pearls.
Because there can be a difference in the type of luster exhibited overall by a category of pearls, when judging luster quality, be sure you are comparing the same kinds of pearls. Akoyas should be judged against Akoyas, and so on.
2. Surface Quality
Surface quality refers to the number and location of “abrasions, bumps, chips, circles, cracks, flat areas, gaps without nacre, pits, scratches, light or dull spots or wrinkles,” according to Gemworld International’s Reference Manual. Judging surface quality involves not only looking for these characteristics, but determining what percentage of the surface they cover. Location is also important. Blemishes near the drill hole detract less from the quality of the pearl than blemishes on the side of the pearl.
Size, measured in millimeters, affects value because larger pearls are rarer. The size of a pearl is affected by both the size of the oyster, which determines the size of the shell bead the producer can implant into the mollusk, and the time in the water, which determines how thick the nacre will be or how large a freshwater pearl will be. Traditionally, Japanese Akoya pearls have rarely been over 7 mm. Tahitian and South Seas pearls have rarely been under 12 to 14 mm.
The most valuable pearl shape is round, as it is the rarest. Akoyas should be round because they’re nucleated with round beads and the nacre is relatively thin. (Akoya oysters are small, requiring small bead nuclei.) However, longer time in the water may cause the nacre to build up unevenly, producing tails or wings on a round bead-nucleated pearl.
Not all producers emphasize roundness. Symmetrical drop shapes in South Seas or Tahitian pearls are very highly valued. Shape value can be affected by market demand. Depending on the fashion, baroque or other pearl shapes might grow in demand among consumers. GIA recognizes round, near round, oval, drop, button, semi-baroque and baroque. However, Chinese freshwater pearls include shapes such as corn or potato, and South Seas and Tahitians include the circlé or ringed pearls. There are other shape names as well.
Color refers to the body color of the pearl, such as white, cream, gray, silver, black, pink, and so on. Body color should not be confused with overtone, which can be easy to do with the strong overtones present in Tahitian pearls. White has been the traditional “best” color for pearls, but the introduction of black, gray, and bronze Tahitians, golden South Seas, and multicolored Chinese freshwater pearls have challenged that perception. What the “best” color is may depend on personal taste or the fashion of the moment.
However, the color should be consistent in each pearl, not spotty. Pearl color can be altered by bleaching, dyeing, and irradiation; virtually all Akoyas are so treated.
6. Nacre Thickness
Pearl nacre thickness or quality affects pearl value because bead-nucleated pearls with very thin nacre are subject to chipping, especially around the drill hole. Nacre thickness is described in millimeters—usually fractions of millimeters.
Japanese Akoyas once stayed in the water for up to three and a half years to allow the pearl to develop a nacre thickness of up to 1.5 mm. But as water quality decreased—and market pressures increased—the Japanese producers shortened the length of time the oyster spent in the water to as little as six months. That resulted in some very poor quality pearls.
Other growers produce pearls with thicker nacre. The French Polynesian government has mandated that Tahitian pearls with less than 0.8 mm nacre thickness cannot be exported; many Tahitians have nacre up to 2 mm thick. South Seas pearls, according to the Paspaley website, may have nacre as thick as 6 mm. GIA has determined that approximately 1.0 mm is the minimum nacre thickness that should be present on a South Seas or Tahitian cultured pearl to ensure durability.
However, freshwater pearls are nacre almost throughout, which puts them in a different category.
Matching is important when evaluating the quality of a pearl strand, bracelet, or set. Pearls should match in color, overtone, luster, shape, and size. Naturally an excellent match is less likely on a commercial strand. If the strand is made up of commercial-quality pearls, poor match will have less effect on the value than it would have on a stand of high-end pearls. This is assuming that the pearls in the strand are supposed to match. Today, it’s fashionable to mix colors and overtones in a single strand. In that case, you would look for matching size, shape, and luster.
Sharon Elaine Thompson is a GG and FGA who has been writing about gemstones and jewelry for Lapidary Journal Jewelry Artist since 1987. She also writes a line of birthstone romance novels under the name Liz Hartley.