Pearls are formed in shellfish, as a reactant to an irritant such as a piece of sand. Cultured pearls are created by adding a piece of mussel or shell inside of an oyster of mussel. This then creates a pearl as layers of the inside of the shell grow over the added substance. The price of pearls vary widely as a result of luster, size, how they were grown, color and type. Natural pearls are the most expensive, followed by cultured pearls. Fresh water pearls tend to be relatively inexpensive. Pearls are generally white, brown, silver, cream, black or pink depending on the type of shellfish and water. Pearl has a hardness of 3.