Certain bivalve (two-shelled) mollusks, like some mussels and oysters, create pearls as a reaction to an irritant that has found its way inside the shell. In nature, this irritant may be microscopic, such as a parasite. The mollusk secretes nacre, the same material used in shell-building, and over time the nacre builds up to create a pearl. Natural pearls may not necessarily be perfectly round. The longer the mollusk works to create the pearl, the greater the chances of its being misshapen or pitted. Both saltwater and freshwater mollusks make pearls.