There are three types of pearls - natural, cultured and imitation. A natural pearl, sometimes called an Oriental pearl or Abalone pearl, forms when an irritant such as a piece of sand works its way into a particular species of oyster, mussel, or other mollusk. As a defense, the mollusk secretes a fluid, called the nacre, to coat the irritant. Layer upon layer of this coating is deposited on the irritant until a pearl is formed.

A cultured pearl undergoes the same process. The only difference is that the irritant is a surgically implanted bead, called a nucleus, or piece of mother-of-pearl pearl. This nucleus is much larger than that in a natural pearl, which results in a thinner nacre than for a natural pearl. This can result in less luster.

Pearls can come from either salt or freshwater sources. Saltwater pearls tend to be higher quality, although there are several types of freshwater pearls that are considered high quality too. Freshwater pearls tend to be irregular in shape, with a puffed rice appearance.