Pearls, whether natural or cultured, are made in the bodies of some species of oysters and are
composed of the same material as the iridescent mother of pearl that lines oysters’ shells, which is a
form of calcium carbonate. If something, such as a grain of sand, gets into an oyster’s shell, it will not be
able to expel it and instead coats it with layers of mother of pearl, creating a spherical pearl. Because
natural pearls are rare, people make cultured ones by inserting a grain of sand into oysters in order to
force them to create pearls, which are harvested after two or three years. These are usually lower in
quality than natural pearls and often not perfectly spherical. Because of this, and because they are less
rare than natural pearls, cultured ones are less expensive to buy.