How does a bivalve create a pearl?

Natural processes can cause almost any type of shelled mollusk to form a "pearl" when a bothersome microscopic object gets caught in the mollusk's mantle folds, but the vast majority of these "pearls" are not prized as gemstones.

The most well-known and economically significant type of pearls are nacreous pearls, which are mainly produced by two groups of molluscan bivalves, or clams.

The same living organism that secretes the mother of pearl lining the shell also creates the layers of nacre that make up a nacreous pearl.

When an irritant such as a grain of sand or a parasite gets inside the shell of a bivalve, it secretes layers of nacre, also known as mother-of-pearl, around the irritant to protect itself. Over time, these layers build up and form a pearl.

A bivalve creates a pearl in response to an irritant, such as a parasite or a piece of debris, entering its shell. In defense, the bivalve secretes layers of nacre, also known as mother-of-pearl, around the irritant. Over time, layer upon layer of nacre is deposited, forming a pearl.