Perhaps the most famous ocean or freshwater jewelry component is not actually a shell but a Pearl.
Pearls come from mollusks (oysters & mussels) and start their life as a grain of sand. The sand irritates the oyster and it creates a layer of nacre that covers the sand particle. The mollusk continues adding more layers and over time you get a natural pearl. Large round natural pearls or pearls that are created wild are rare and highly treasured. Most pearls today come from pearl farms and are known as cultured pearls.