What substance forms most mollusk shells?

Most mollusk shells are formed primarily from calcium carbonate.

The mineral is deposited by the mantle, a layer of tissue bordering the mollusk’s soft body. The mantle secretes calcium carbonate crystals within an organic framework of proteins and polysaccharides. Depending on the species and shell layer, the crystals may take the form of calcite or aragonite. This combination produces a hard protective exoskeleton while allowing intricate shapes, colors, and textures.

Many shells have an outer prismatic layer and an inner nacreous, or pearly, layer. Nacre is made largely from aragonite tablets arranged in an organic matrix, which gives pearl oysters and some other mollusks their shimmering appearance. A leathery outer coating called the periostracum helps protect the growing edge.

Not every mollusk retains a large shell: squid and octopuses have reduced or internal shells, and some mollusks have lost shells entirely. Another common mistake is to choose chitin. Chitin can occur in the organic shell matrix of some species, but it is not the main mineral substance of ordinary mollusk shells.