According to my chemistry book (Chemistry: The Molecular Nature of Matter and Change by Silberberg) seashells including clam shells are primarily composed of Calcium Carbonate.
Mantle
The mantle of a clam protects the clam from sunlight in shelled mollusks, the mantle is the organ that forms the shell, and adds to the shell to increase its size and strength as the animal grows.
The mantle in a clam is a thin layer of tissue that surrounds the body and lines the inside of the shell. It is located between the body and the shell of the clam and plays a role in secreting the calcium carbonate that forms the shell.
A clam shell
Possessive form of the words clam shell: clam shell's
The hinge ligament is a flexible structure that connects the two halves (valves) of a clam shell at the hinge. It acts as a spring to allow the shell to open and close. When the adductor muscles relax, the hinge ligament helps the clam's shell to open.
The umbo, also known as the beak, is the oldest part of a clam shell. It is the point where growth starts and forms the center of the shell. Over time, new layers of shell material are added, extending outwards from the umbo.
A clam shell is not a mineral because it is composed of organic material (calcium carbonate) secreted by the clam itself, rather than forming through geological processes like minerals do. Minerals are naturally occurring inorganic substances with a specific chemical composition and crystal structure, which a clam shell does not meet the criteria for.
A clam. Or an oyster.
The shell of a clam is primarily made of calcium carbonate. This compound gives the shell its hardness and strength to protect the soft body of the clam.
A clam shell grows via a process where tissue is attached to the shell at the edge. Over time this material builds up producing a ring like pattern. The number of rings will indicate the age of the clam shell.
The mantle of the clam lines the valves, and serves to secrete calcium carbonate in the formation of the clam's shell while protecting the clam's visceral mass.