In general, cultured pearls are less valuable than natural pearls, and imitation pearls are less valuable than cultured pearls. One way that jewelers can determine whether a pearl is cultivated or natural is to have a gem lab perform an x-ray of the pearl. If the x-ray reveals a nucleus, the pearl is likely a bead-nucleated saltwater pearl. If no nucleus is present, but irregular and small dark inner spots indicating a cavity are visible, combined with concentric rings of organic substance, the pearl is likely a cultured freshwater. Cultured freshwater pearls can often be confused for natural pearls which present as homogeneous pictures which continuously darken toward the surface of the pearl. Natural pearls will often show larger cavities where organic matter has dried out and decomposed.
Some imitation pearls are simply made of mother-of-pearl, coral or conch, while others are made from glass and are coated with a solution containing fish scales called essence d'Orient. Although imitation pearls look the part, they do not have the same weight or smoothness as real pearls, and their luster will also dim greatly.
There is also a unique way of naming pearl necklaces. While most other necklaces are simply referred to by their physical measurement, strings of pearls have their own set of names that characterize the pearls based on where they hang when worn around the neck. A collar will sit directly against the throat and not hang down the neck at all; they are often made up of multiple strands of pearls. Pearl chokers nestle just at the base of the neck. The size called a princess comes down to or just below the collarbone. A matinee of pearls falls just above the breasts. An opera will be long enough to reach the breastbone or sternum of the wearer, and longer still, a pearl rope is any length that falls down farther than an opera.
Necklaces can also be classified as uniform, or graduated. In a uniform strand of pearls, all pearls are classified as the same size, but actually fall in a range. A uniform strand of akoya pearls, for example, will measure within 0.5 mm. So a strand will never be 7 mm, but will be 6.5-7 mm. Freshwater pearls, Tahitian pearls, and South Sea pearls all measure to a full millimeter when considered uniform. A graduated strand of pearls most often has at least 3 mm of differentiation from the ends to the center of the necklace. Popularized in the 1950s by the GIs bringing strands of cultured akoya pearls home from Japan, the graduated style was much more affordable as most pearls in any given strand were small.
Earrings and necklaces can also be classified on the grade of the color of the pearl. While white and more recently black pearls are by far the most popular colors other tinges of color can be found on pearls. Pink, blue, champagne, green and even purple can be found, but to form a complete string of same size and shade pearls can take years. Some colors like purple can only be found in certain types of clams, while other clams can produce a variety of colors if given the right environment.