Oysters and mussels are bi-valve molluscs, and the creation of a pearl is a form of defense protecting it against damage from anything entering the shell, another organism, or some other object such as a piece of shell or sand. The mollusc would attempt to eject an intruder by washing it out of its shell. If this fails, it would begin to secrete two complex materials, which combined, is called Nacre and is laid down in layers around the invading object. Once commenced, this layering process continues throughout the life of the mollusc. The result is the formation of a Natural Pearl.
Before the 1930's virtually all pearls would be classified as Natural Pearls. For thousands of years the most important fisheries were the Persian Gulf, and the Straits of Manaar at the tip of India. However these usually small, often irregular shaped pearls were extremely rare, and therefore commanded high prices. Due to their rarity and enormous prices, pearls were the gems of royalty and the aristocracy, and today the pearl's understated aura of elegance is the legacy of these times. Very few of these pearls were round, and therefore a round pearl was the most highly sought after and the most expensive.
The escalating pearl prices in the late 19th/early 20th century, followed by the Depression of the 1930's led to a fall in confidence and a collapse of natural pearl prices; the fishing for natural pearls ceased in the early 1930's, and has never been revived. Cellini still has strands of Natural pearls dating from the early twentieth century.
The late 1920's saw the commercial establishment of the Japanese cultured pearl industry, a contributory factor in the demise of the natural pearl trade.