Pearls are formed in shellfish, as a protective reaction to irritants such as sand. Cultured pearls are created by adding a substance like a piece of mussel or shell inside of an oyster of mussel. This then creates a pearl as inside layers of the shell grow over the substance. The price of pearls vary widely as a result of luster, shape, size, how they are grown, color and type. Natural pearls are more expensive than the cultured ones. Fresh water pearls are usually less expensive. Pearls are white, brown, silver, cream, black or pink depending on the type of shellfish and water. For thousands of years, most seawater pearls were collected by divers in the Indian Ocean, Persian Gulf, Red Sea, and Gulf of Mannar. The chinese hunted extensively for seawater pearls in the South China Sea. Today, most pearls used for jewelry are cultured.