Pearls start when an irritating microscopic object becomes trapped within the soft tissue of a live mollusk. The mollusk creates a pearl sac to seal off the irritation and then secretes multiple layers of nacre (calcium carbonate), eventually creating a pearl Ideally, pearls are round and smooth, but many other shapes can occur due to the environment in which the pearls develop. Fine quality natural pearls have been highly valued as gemstones for centuries.
Gem-quality pearls are almost always nacreous and iridescent, like the interior of the shell that produces them. A nacreous pearl is made from layers of nacre, by the same living process as is used in the secretion of the mother of pearl which lines the shell. All types of pearls are quite “soft” and range between 2.5 and 4.5 on the Mohs scale of hardness.
There are many types of pearls:
Natural pearls are pearls that occur without any human intervention. Today a lot of pearls are farm raised – either freshwater or salt water. Many cultured pearls are sold by their trade names: Akoya, black Tahitian, etc.
Cultured pearls are created when a foreign substance is intentionally inserted into a living oyster or mollusk and most cultured pearls are farm raised in China today. Cultured pearls can be distinguished from natural pearls using gemological x-ray techniques. Imitation pearls are completely smooth, but natural and cultured pearls are composed of nacre platelets, making either feel slightly gritty.
Baroque pearls (irregularly shaped),
Biwa pearls (irregularly shaped pearls formed in the freshwater of Lake Biwa, Japan),
Blister pearls (half-spherical pearls which grow attached to the inside of the shell), Mabe pearls (cultivated blister pearls)
Keshi pearls are considered a byproduct of the culturing process. They can be either freshwater or saltwater: Keshi pearls are produced by many different types of marine mollusks and freshwater mussels in China. Keshi pearls are entirely nacre.
The unique luster of pearls depends upon the reflection, refraction, and diffraction of light from the translucent layers. The more numerous the layers in the pearl, the finer the luster. The iridescence that pearls display is caused by the overlapping of one layer on top of the next, which breaks up light falling on the surface.
Pearls vary in color and can be white or with a hint of color ranging from pink to brown or black. The color depends on the type of mollusk and the water where the mollusk lived. Freshwater pearls can be dyed. Creating a pearl can take seven or eight years. Pearl is the official birthstone for the month of June.