A clam pearl is a generic description of gems found inside certain bivalves located throughout the world. Clam pearls in different colors and patterns represent those most valued by collectors. They can grow quite large, but are usually smaller than saltwater pearls found in oysters. A clam pearl typically is not completely round and lacks the high luster found in cultured pearls.
Also called river pearls, clam pearls usually come from freshwater bivalve mussels that live in rivers and streams. The mussel as a food source goes back to native people who regularly ate the soft-bodied mollusk. Clam pearls were collected by Native Americans as treasured ornaments that were worn and traded.
A clam pearl contains a thin coating of a substance similar to calcium carbonate. The mussel secretes this substance to coat a foreign irritant that enters the shell during feeding. Clams open and close two hinged shells to feed, but sometimes a parasite or other foreign body is not filtered out and irritates the soft body of the mussel. Over time, the calcium carbonate secretions develop into a pearl.
Cultured pearls are generally more valuable than clam pearls because of their perfectly round shape and luminosity. A clam pearl might be found in different colors, but white is the most common. Other colors range from pink to lavender to a rare blue clam pearl sought by collectors. The color of a clam pearl is determined by the kind of mussel that produces it.
Littleneck clams found along the eastern coast of the Untied States produce quahog clam pearls with various shades of purple. These irregular saltwater pearls quickly became popular for their distinctive colors. They might be valued for the irregular shapes that differ so much from the more expensive cultured pearls.
The shape of a clam pearl depends on where it is found inside the shell. Some form on the lip of the bivalve, which tends to make them more round. Others that might be shaped like a wing develop on the back of the shell. Most clam pearls are attached to the heel of a shell, which accounts for their irregular shapes. A perfectly round clam pearl is quite rare.
Clam shells were collected and used in pearl button factories in the past. At one point, there more than 40 button manufacturers in Iowa along the Mississippi River in the United States. Factory workers cut circles from the shells, which were ground down and polished with water and pumice stones. Holes were drilled in the final product to create buttons used in the clothing industry.