Oyster farm. Diver examining a basket bearing oysters at a pearl farm in the Tuamotu archipelago. This region is renowned for its black pearls. These were once highly prized due to their rarity but this has changed as they are now cultivated. A pearl is produced by an oyster as a response to an irritant (such as a sand grain) in its shell. The oyster secretes layers of nacre around the irritant, eventually forming a pearl. Cultured pearls are made by keeping the oysters in a protected environment and manually introducing an irritant into their shells. They are tended to on a daily basis and the resulting pearls are harvested years later. Photographed in French Polynesia, Pacific Ocean.