As a defense mechanism, the mollusk secretes a fluid to coat the irritant. Layer upon layer of this coating is deposited on the irritant until a lustrous pearl is formed. A cultured pearl undergoes the same process. The core is, therefore, much larger than in a natural pearl. As long as there are enough layers of nacre to result in a beautiful, gem-quality pearl, the size of the nucleus is of no importance to beauty or durability.
Imitation pearls are a different story altogether.
In most cases, a glass bead is dipped into a solution made from fish scales. This coating is thin and may eventually wear off. The island of Mallorca is known for its imitation pearl industry. Pearl Waters Fine natural pearls are quite rare. The Persian Gulf has always been the source of the finest natural saltwater pearls.