Pearls are some of the most beautiful gems we know, and very expensive to buy. So it may come as something of a surprise to learn that pearls start off as irritating grains of sand and other substances which get into the soft part of the oyster. The oyster is a mollusc, which means that it has a soft and very sensitive body inside its hinged shell. When a grain of sand or grit gets inside the shell and on to the soft body of the oyster, the creatures covers it with layer upon layer of a hard smooth substances called nacre, which is the same substance which makes mother-of-pearl. It does this to cut down the irritation caused by the sand, and it adds more and more nacre, forming the pearl. It is easy to see how people hit on the idea of deliberately putting a grain of sand or a small bead inside the oyster and allowing the animals to cover it with nacre, so that oyster forms what is called a cultured pearl; although these are real pearls, they are started deliberately.