Pearl Formation

The shells of mollusks are secreted by a part of the body which is termed the mantle. The surface of the mantle consists of a layer of cells which is continually growing and excreting shell substances.

The bivalve shell consists of three layers:

• A. A thin outer layer of dark material called conchiolin;
• B. A thicker, crystalline layer called calcite prisms;
• C. An inner layer consisting of conchiolin plus aragonite platelets. This is mother of pearl or nacreous layer.

The nacre of pearl is also in the form of aragonite plus conchiolin, which is secreted by the same part of the mantle that produces the nacreous layer of the shell.

The formation of a natural pearl is the result of an accidental occurrence within the normal life cycle of a mollusk.

The picture below shows the formation process of pearl: