Pearls were largely an accident of nature until the late 19th century, when entrepreneur Kokichi Mikimoto invented a method of cultivating them by introducing a foreign body into the shells of oysters.
A shell protects a mollusk's organs from predators. The first of three separate components of a mollusk's shell is an inner layer of nacre (mother-of-pearl), which is composed of a continuously secreting calcium carbonate that is used to smother foreign objects. This sustained reaction ultimately produces pearls.
After receiving a patent for "nucleated" pearl production in 1916, Mikimoto eventually produced a saltwater pearl from the diminutive Akoya oyster that is native to Japan. Akoya pearls are still highly valued today.