"Pearls are an organic gem created by oysters, clams, and mussels, mollusks that are found in both freshwater and marine habitats. Pearls harvested from saltwater are sometimes referred to as Oriental pearls, while those from freshwater are called freshwater or river pearls.
A pearl is an abnormal growth that results when a foreign object, such as a grain of sand, invades the soft inner tissues of a mollusk's body. The particle acts as an irritant inside the mollusk and is coated with layers of material called nacre, or mother-of-pearl, which is secreted by the animal's shell-secreting cells. Nacre is composed of aragonite, a form of calcium carbonate, and also contains a small amount of conchiolin, a horn-like organic substance (albuminoid) that is the main component of the mollusk's outer shell.
These gems are characterized by their translucence, luster, and the delicate play of surface color called orient, which is the soft iridescence caused by the refraction of light by the layers of nacre. The more perfect the shape (spherical or drop like) and the deeper its luster, the greater the value of a pearl. Pearls are found in many colors but the most prized are white, black, cream, and rose -- the Indian rose pearls being the most esteemed. Color varies according to the species of mollusk and its environment."
