The outer protective layer of a mollusk shell is called the periostracum. This layer is primarily composed of a protein called conchiolin, which gives it a tough and leathery texture. The periostracum acts as a shield, protecting the inner layers of the shell from environmental damage, such as erosion and predation.
Beneath the periostracum lies the prismatic layer, which is made up of calcium carbonate crystals. This layer provides additional strength and structure to the shell. The innermost layer, known as the nacre or mother-of-pearl, is smooth and iridescent, often used in jewelry and decorative items.
Together, these layers work to protect the mollusk and provide structural integrity to its shell, allowing it to thrive in various aquatic environments.