Pearls That Were Cultured

Pearls are formed naturally inside oysters(牡蛎) .But only one oyster in a thousand produces a pearl large enough to be a precious jewel.Oysters produce pearls to protect themselves. Sometimes a tiny piece of sand gets inside an oyster's shell (壳) .It could harm the soft flesh of the oyster.So the oyster covers the piece of sand with a layer (层) of smooth shell material .Each year new layers are added,and the pearl gets bigger.