Oyster shells are the protective outer covering of oysters, a group of bivalve mollusks found in coastal waters. Each shell has two valves joined at a hinge, with a hard, layered structure that grows as the oyster does. The valves are often uneven and irregular in shape, and their surfaces may be ridged or rough. Oysters can attach to firm surfaces and to one another, forming reefs that provide habitat for other marine life.