Pearls are unique, and so are many of the ways in which people determine their quality and rarity. While color and size play a role, just as they do with gemstones and diamonds, other characteristics also come into play.
Types of Pearls
Before considering the characteristics of pearls, let's look at the different types that are available today. All of these are cultured pearls, created by stimulating an oyster or mussel to produce a pearl. Natural pearls are so unusual today that they are rarely available for sale.
South Sea pearls are very rare. Exceptionally large, smooth, and round, they may be white/cream, golden, or black. They come from white-lipped oysters in the waters of Australia and Southeast Asia.
Tahitian pearls, traditionally known as black pearls, get their color from the giant black-lipped oysters in which they grow. Slightly smaller than South Sea pearls, they are cultivated exclusively in the volcanic atolls of Tahiti.
Akoya pearls are the pearls that often come to mind when people think of classic white pearls. Cultured in saltwater mollusks in Japan and China, they look similar to freshwater pearls, but tend to be larger and more lustrous.
Freshwater pearls come in shades from white to black, depending on the type of mollusk that grows them. They may look at a glance like Akoya pearls, but they are generally smaller and less symmetrical - and therefore less valuable.
A pearl's color is actually a combination of two types of color—the body color and the overtone. Some of the most desirable pearls have overtones that are equal in depth to their body color.
Body color refers to a pearl's primary color. Most people think of white or cream-colored stones when they think of pearls, but they appear in many other shades as well, such as pink, rose, silver, blue/gray, gold, and black.
Overtone refers to secondary color - typically rose, pink, or silver - that appears as light reflects off the surface of a pearl. For example, a white pearl may have a pink overtone when seen under the light.
Size is not the only criteria by which to judge the quality of a pearl, but if all other factors—color, luster, shape, and surface—are the same, then it's fair to say that the value of a particular pearl will be higher because of its size. Different types of pearls have different size ranges, and these sizes are measured in millimeters.
South Sea and Tahitian pearls generally go up to 13 mm or so in size; however, some as large as 20 mm have been found.
Akoya pearls range from about 6.0 to 8.5 mm.
Freshwater pearls range in size from about 3.0 to 7.0 mm.
Luster refers to a pearl's shine, or the effect that's created when light is reflected off the iridescent material called nacre that composes a pearl. The larger the pearl, the more nacre—therefore, the more luster. And the higher the luster, the more valuable the pearl.
Very high and high are the grades of luster that refer to extremely bright, sharp light reflections. Because this is so desirable, it can make up for sub-par size, color, shape, or surface.
Soft or dull is the grade that refers to weak or imprecise reflections on the surface of the pearl. It is characteristic of the lowest-quality pearls.
Medium grade is somewhere in the middle. Pearls at this grade are less costly than those graded very high or high in luster, but more desirable than those graded soft.
While pearls come in several different shapes, round pearls are the most desirable. The rounder the pearl, the higher the quality. A perfectly round pearl is extremely rare.
Round is defined as a pearl whose rate of diameter varies by less than 2%.
Semi-round pearls have a greater diameter variation than round pearls, but still less than 5%.
Drop, button, pearl, and oval-shaped pearls are called semi-baroque or baroque pearls. Semi-baroque pearls have an axis of rotation, which means you can actually spin one on a tabletop.
The ideal pearl has a smooth surface that is relatively free of cracks, discoloration, or other imperfections. This quality is judged by experts who examine pearls with the naked eye to determine surface ratings.
Clean, or very lightly blemished, is the highest rating for the surface quality of a pearl, and is typical of the most desirable pearls.
Lightly or moderately blemished pearls are less desirable.
Heavily blemished pearls are the least desirable.