Pearls form when a particle enters a mollusk's shell by accident, acts as an irritant, and a material called nacre is formed in layers around the particle. Pearls formed without any human intervention are called natural pearls and are very rare.
The most commonly used pearls in jewelry are cultured pearls. The principle of a particle entering a mollusk shell and forming nacre is the same, however, in the case of cultured pearls, it is man who introduces the irritant. Mollusks are removed from the water, a foreign substance in put into their tissues, and they are returned to the sea so the pearls develop. Cultured pearls are cultivated in both fresh and salt waters:
Freshwater (shown in pearl and amethyst necklace): from China and US. Many shapes and sizes, abundant and popular, inexpensive to moderately priced
Tahitian: saltwater, also called black pearls (they are not really black but are usually shades of green, blue, grey, or purple, etc.) very expensive, only pearls grown in French Polynesian islands may be called Tahitian pearls
South Sea: saltwater, large, round, white, expensive, mostly from Japan but from other Asian countries as well
Akoya: saltwater, small, round, brilliant color and luster, expensive, from Japan, other Asian countries, and Australia, used often in classic pearl jewelry
Mother of pearl (shown in fan shaped pearl earrings): the iridescent shell lining of various mollusks. It has many irregularities and is less expensive than pearls. However, it is used as inlay, pendants, as well as various bead shapes can be very gorgeous jewelry component.
Man made (shown in pearl and wire wrapped pendant): (also referred to as imitation, simulated or faux pearls) have a crystal or glass core with a pearl coating. They are uniform in shape, often round, and come in a dazzling array of colors and sizes. In my opinion, the most beautiful faux pearls are made by two companies - Swarovski and Preciosia crystals. There are also faux pearls made of plastic or cotton and can really be fun on some types of fashion jewelry.
