Why do clams have pearls?

Answer

Clams are not actually known as the primary producers of pearls although they can and do produce pearls. Clam pearls are known as calcareous concretions, not as nacreous pearls. Clams do not produce nacre. Mollusks from the family Bivavlia are the primary pearl producers, especially mollusks from the genus Pinctada.

Contrary to popular belief, a pearl is not created by a grain of sand. This is a widely accepted urban legend. A pearl is most often created when a parasite burrows through a mollusk's shell and into the mantle tissue of the host. The defensive reaction of the mollusk is to grow a sac around the intruder. This is known as the pearl sac. The lining of this sac is made up of nacre-producing cells known as epithelial cells. These cells produce a chemical compound of aragonite and calcite (a calcium carbonate polymorph) which is laid down upon the intruder in the form of platelets. These platelets are commonly known as nacre. Nacre is the pearl.

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