Formation of pearls
Natural pearls are formed when a foreign object enters a mollusk’s shell. To defend it, layer after layer of a substance called nacre grow and form like onion skins around the particle. Those are semi-opaque layers that consist of the calcium carbonate that makes up the pearl and becomes a coat for the particle. Inevitably, natural pearls vary in shape because the process depends on the shape of the piece being coated by the mollusk. They are very rare and expensive.
Therefore, a process has developed to imitate the process of natural pearl formation that produces pearls that are called “cultured”. Cultured pearls are sold by their sizes in millimeters. Like natural pearls, cultured pearls grow inside of a mollusk, but with human intervention. Shells are carefully opened and different shapes of beads are inserted. Over time, the inserted beads become coated with nacre, which makes a pearl appear to glow inside and gives it a beautiful shine.