Pearls are classified by origin, then graded by size, shape, nacre thickness, color, luster, surface clarity, and matching. These qualities are not considered equal. Some factors will be weight to give them influence in arriving at the final grade. A very thin nacre thickness; for example, could never yield a fine quality Pearl.
Grading is relative to the best attainable quality for the type of pearl. For example, South Sea Pearls, which grow in warmer water for longer periods of time, generally have a lower luster and more tiny blemishes than Japanese Akoya pearls that grow in colder water and for shorter periods of time. South Sea pearls are graded against each other, not by what would be expected for a similar quality Akoya pearl.
Luster is the deep inner glow of the pearl and its brilliance to the human eye. For cultured pearl experts, this factor is perhaps the most important indicator in evaluating cultured quality because it is what separates the ordinary from the extraordinary. Throughout history, this unique quality has separated cultured pearls from all other gems and it is what many experts term the heart and soul of a cultured pearl.
Surface quality refers specifically to the abundance of physical blemishes. When evaluating surface (the trade uses such terms as blemish, spotting, and cleanliness), remember that cultured pearls are grown by live oysters in nature. As such, there are many uncontrollable forces that affects the surface. The fewer the natural markings or spots on the pearl’s visible surface, the more expensive the pearl.
Shapes range in descending order of value from round to semi-round, from off-round to oval and from drop to baroque. Generally the shapes from round to drop are pretty symmetrical, while anything baroque denotes a pearl that is completely asymmetrical or free-form. The more spherical (rounder) and symmetrical the pearl, the more valuable it will be. Irregular shaped cultured pearls can be very attractive and are usually less expensive than round cultured pearls.
Cultured pearls come in a variety of colors from silvery white to black, with a rainbow of colors in between. This includes body colors (the overall color) and overtones (translucent color that appear over the body color). Color is entirely a matter of personal preference. Because of changes in the natural habitat, the availability of certain colors can vary; and scarcity affect cultured pearls prices.
Cultured pearls are measured by their diameter in millimeters (mm). They can be smaller than one millimeter in the case of tiny seed pearls, or as large as twenty millimeters for a big south sea cultured pearl. The larger the pearl, other factors being equal, the more valuable it will be. Although this does depend on the type of the pearl, for example, if an extremely large Akoya cultured pearl is discovered it may be more valuable than an average sized South Sea cultured pearl.
Matching cultured pearls refers to pairs or strands, and addresses uniformity of color, luster, shape, spotting and graduation. The more uniform and aesthetically pleasing two or more pearls look together the more time was spent matching the pearls. This time to match cultured pearls is reflected in the cost. If colors are intentionally mixed, it refers to the attractiveness of the combination. Each pearl in a strand has been selected to be placed between its neighbors on each side, and there should be no noticeable differences between pearls that are side-by-side.