Pearls are the result of an irritant intruding the shell of a host mollusk, usually a mussel (various oysters) but sometimes a snail. The host secretes mother-of-pearl (also called nacre), the coating used to cover the inside of its shell, to cover the irritant. This coating thickens over time and a pearl is created. Most Pearls currently used are Cultured Pearls in which the irritant, a bead made out of clam shell, is inserted by a person. Natural Pearls are extremely rare and therefore very valuable.
Pearl is the June birthstone.

Pearls used in jewelry are divided into three major categories; South Sea Pearls, Akoya Pearls and Freshwater Pearls.

South Sea Pearls

South Sea Pearls are cultured in the warm southern seas of Tahiti, Australia, Indonesia, the Philippines and a few other locations. The oysters from which South Sea Pearls are harvested are as much as four times larger than the oysters which produce Akoya Pearls. South Sea oysters secrete more nacre, thus producing larger pearls (usually 9-17mm) which have a superior luster. The size of the oysters combined with their rarity, the difficulty in cultivating them and the limited viable harvesting area result in South Sea Pearls being the most valuable of the three major types of Pearl.

South Sea Pearls are divided into two groups based on color and origin: The White Group includes Pearls harvested mainly in Australia, Indonesia and The Philippines. South Sea Pearls from the White Group are found in colors from white to cream to gold and silver.

The Dark Group are mainly harvested in Tahiti and are sometimes called Tahitian Pearls. South Sea Pearls from the Dark Group range in color from black to gray to peacock green, green, and blue.

Akoya Pearls are the most common type of pearl used in jewelry. The vast majority of Akoya Pearls are cultivated in the coastal waters of Japan. These Pearls usually vary in size from 3 to 9mm and are predominantly round in shape.

Akoya Pearls are usually white or cream colored and have additional faint colors called overtones which are typically pink, gold, silver, blue or green.

The cost of Akoya Pearls depends on availability, size and quality. The quality of an Akoya pearl is judged based on the shape (perfectly round being most valuable), color, luster and the texture and cleanliness of the surface.

Freshwater Pearls are the most plentiful type of pearl. Unlike South Sea and Akoya Pearls, most Freshwater Pearls do not have a mother-of-pearl bead, instead small slivers of tissue are inserted into the mussel. Because of this, Freshwater Pearls are as much as 98% nacre. Also unlike Akoya and South Sea Pearls, the mussels used to produce Freshwater Pearls can create more than one pearl at a time. Some can create as many as 60 at one time. Freshwater Pearls also grow much faster than other types of pearls.

Freshwater Pearls are found in a variety of shapes, the most common being oval, egg, button and drop. They are found in a wide variety of colors including white, champagne, cream, orange-pink, purple, mauve, dark blue and brown.

Because of their relatively easy and quick harvest and the production of multiple pearls in each mollusk, Freshwater Pearls are the most plentiful and also least expensive type of pearl.