Real pearls are created in oysters, so short of putting a bed of these marine workers on your payroll, it can be a little difficult to control the quality of pearls you are going to get, and this is what makes them so valuable. Natural pearls are very rare, so most pearls that you find on the market today are cultured, which means oyster farmers have implanted beads that the oyster then coats. While there is not an internationally accepted grading system, there is a list of factors that determine a pearl’s worth. When inspecting your pearl, be sure to use natural light or a cool-white fluorescent light. This gives you the best opportunity to spot imperfections.
The type of pearl you get depends on the species of oyster, the body of water it is from, the region where the oyster lived and the climatic conditions. Akoya pearls are quite rare because the oyster cannot survive when its pearl is harvested, and therefore its pearls are more valuable. Other oysters, particularly those from fresh water, can be harvested multiple times and so produce more pearls.
Most pearls are creamy in color, while others have silver, champagne, gold, green, blue and pink overtones. Pearls are actually measured on two variables of color: the body color of the pearl is referred to as its basic color, and the overtone is it’s secondary tint. These two color variables are usually not the same; for example, black pearls usually have pink or green overtones.
Pearl size is measured in carats for natural pearls and by diameter (in millimeters) for cultured pearls. For most pearls, if all other factors are the same, the larger it is, the more expensive. Akoya pearls are usually between 6 to 8 mm and freshwater, South Sea and Tahitian pearls can be larger. Seed pearls are 1 mm or less.
Nacre is the substance an oyster creates to cover an irritant within the shell, and this is what becomes the outer coating of the shell. Nacre is multiple layers of calcium carbonate crystals. Generally, the thicker the nacre, the more the pearl is worth. Because cultured pearls are often made by inserting a bead into an oyster, which it then covers with a thin coat of nacre, natural pearls with thicker nacre are typically more valuable.
Luster is actually a result of the nacre itself, and is measured by the pearl’s brilliance and reflectivity when light reflects off the layers of calcium carbonate crystals. Bright and shiny pearls are worth the most, while those with a chalky, dull appearance command less value. Usually, saltwater pearls are more lustrous than the freshwater variety.
As a pearl forms inside a mollusk, the layers of nacre may not bond smoothly, creating bumps, indentations, bubbles and discolorations. The most valuable pearls have the fewest imperfections, although flawless ones are extremely rare.
Though round pearls are the most popular on the market, it’s difficult to find those that are perfectly spherical. Akoya, Tahitian and South Sea pearls are generally the roundest, while freshwater pearls are typically off-sphere oval-shaped. Other pearls, which are generally not as expensive, are flattened button pearls, teardrop-shaped drop pearls and asymmetrical baroque pearls.