Pearls are hard nodular precipitates formed inside the shells of mollusks such as clams.
They consist of layers of calcium carbonate, either in the form of calcite or aragonite held together by a horny organic substance, such as conchiolin. The hard iridescent material with the typical pearly luster is called nacre. Pearls usually grown around some sort of nucleus, such as a sand grain, that irritates the tissues of the clam and caises the carboate precipitation. Pearl-bearing clams occur in many lakes and rivers of Wisconsin. The collection and transformation of these valuable natural pearls into jewelry makes for a small cottage industry in the state.