What is the nacreous layer inside a pear-bearing mollusk's shell commonly called?

The nacreous layer inside a pearl-bearing mollusk's shell is commonly referred to as "mother-of-pearl." This term specifically denotes the iridescent inner layer of the shell that is composed of layers of nacre, or aragonite and conchiolin. This layer is formed by the mollusk as a protective mechanism against irritants, such as parasites or debris. When a foreign substance enters the mollusk, it secretes nacre around it, eventually leading to the formation of a pearl.

Mother-of-pearl is prized not only for its beauty but also for its use in jewelry and decorative items due to its lustrous appearance and unique colors that change with the angle of light. Nacre itself refers to the same material, but the term "mother-of-pearl" is more commonly used in consumer contexts and can evoke imagery of luxury and refinement, making it a favored term in the industry.