How do pearls grow in water?

Pearls grow in pearl-producing mollusks. Natural pearls occur when a mollusk's mantle tissue becomes damaged either by parasitic intrusion or damage to the shell. The mantle contains epithelial cells, which are responsible for the nacre deposition on the inside of the shell. If these cells become dislodged when the mantle is damaged, they may continue to deposit nacre within the body of the mollusk, creating a pearl.

A grain of sand neverproduces a pearl. This is a myth.