Pearls come from mollusks-a group of animals, which includes squids, clams, and oysters.
Under its shell the mollusk has a mantle, or outer-skin. When a particle of dirt, or tiny marine creature gets stuck in this mantle, a hard substance forms around it. This makes the pearl. It is formed of the same material as the mollusk shell-nacre or mother-of-pearl.
Pearl come in many different shapes and colors. The most valuable ones are completely smooth, spherical and usually white. But there is a black pearl from the Gulf of Mexico, which is also extremely valuable.
The people who first discovered how to stimulate the production of pearls were the Chinese. They put the mollusk into a shallow pond, inserted a speck of mud into the mantle, and waited for the pearl to form. After a couple of months they would detach the pearl from the mollusc’s mantle-and eat the mollusk.