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 causes the carbonate 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. Although organic and not, by strict definition, minerals, they are included because they are an important Wisconsin lapidary material.
CALUMET COUNTY: Pearls are found in clams in various streams and lakes in the county (Eckert, 1980).
CRAWFORD COUNTY: Pearls are found in clams from the Mississippi River in the Prairie du Chien area (Eckert, 1980).
GRANT COUNTY: Pearls occur in clams in the Wisconsin River near Boscobel and in the Grant river and its tributaries near Potosi (Eckert, 1980).
GREEN COUNTY: Pearls occur in clams in the Sugar River near Broadhead.(Eckert, 1980).
LAFAYETTE COUNTY: Pearls occur in clams in the Apple River south of Dunbarton and in the Pecatonia River near Gratiot (Eckert, 1980).
MANITOWOC COUNTY: Pearl-bearing clams occur in water bodies throughout the county. (Eckert, 1980).
PEPIN COUNTY: Pearls occur in clams from the Mississippi River near Stockholm and elsewhere along the Mississippi River corridor in Pepin and adjacent Pierce and Buffalo Counties. Pearl jewelry is currently (1994) marketed by stores in Stockholm (W. S. Cordua notes).
ROCK COUNTY: Pearls occur in clams in the Rock River (Eckert, 1980).