What are hermit crabs shell made of?

Hermit crabs don't actually make their own shells. Instead, they rely on finding and using empty shells of other sea creatures, most commonly snails.

So, a hermit crab's shell is actually made of the same material as the snail shell it's using, which is primarily calcium carbonate (CaCO3). This is the same material that makes up seashells, coral reefs, and even human bones.