The term 'shell material' refers to the material that makes up the outer layer of a mollusk's shell. NACRE, also known as mother-of-pearl, is a type of shell material composed of calcium carbonate and an organic compound called conchiolin. It is secreted by the mollusk and forms a hard, iridescent layer on the inside of its shell. NACRE is highly valued for its beauty and is used in jewelry and other decorative items.