A pearl is a hard, rounded object produced within the soft tissue (specifically the mantle of a living shelled mollusk). The finest quality pearls have been highly valued as gemstones and objects of beauty for many centuries.
Technically speaking, under the right set of circumstances almost any shelled mollusk can produce some kind of pearl. However, most molluscan pearls have no luster or iridescence. In fact, the great majority of mollusk species produce pearls which are not attractive to look at, and not durable, such that they usually have no value at all, except perhaps to a scientist, or as a curiosity.
Desirable pearls are produced by two very different groups of molluscan bivalves or clams: marine pearl oysters from the family Pteriidae and freshwater pearl mussels from the order Unionida, families Unionidae and Margaritiferidae. These two families of clams, although not closely related to one another, have shell linings that are composed of nacre, or mother of pearl as it is more commonly known. A natural pearl is entirely made from layers of nacre, using the same spontaneous process as is used in the creation of the nacre lining the shell.
Although in illustrations a gem-quality pearl is often pictured sitting in an edible oyster shell, this is a mistaken idea. Pearl oysters are not closely related to edible oysters, and edible oysters cannot form real pearls.
Pearls have long been greatly valued as gemstones. They have been harvested, or more recently cultivated, primarily for use in jewelry. Pearls have also, rarely, been crushed and used in cosmetics or in paint formulations. Pearl is sometimes considered to be the birthstone for the spring/summer month of June.
Drop and pear shaped pearls are sometimes referred to as teardrop pearls and are most often seen in earrings, pendants, or as a center pearl in a necklace. Baroque pearls have a different appeal to them than more standard shapes because they are often highly irregular and make unique and interesting shapes. They are also commonly seen in necklaces. Circled pearls are characterized by concentric ridges, or rings, around the body of the pearl. 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 homogenous 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 .5mm. So a strand will never be 7mm, but will be 6.5-7mm. 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 3mm of differentiation from the ends to the center of the necklace.