Pearls are hard objects that are produced in the soft tissues of living mollusks. Like its shell, pearls consist of calcium carbonate in the form of crystals that have been deposited in concentric layers. The finest quality natural pearls have been highly valued as objects of beauty for centuries. Therefore the word "pearl" has become a metaphor for something very rare, good, wonderful, and precious.

A natural pearl is formed without any human intervention, in the wild, and is very rare. Approximately hundreds of pearl shells had to be collected and opened, and thus killed, only to find one wild pearl, and for centuries that was the only way to obtain pearls. This is the main reason why pearls were so valuable in the past. Cultured pearls, on the other hand, are a type of pearl that is formed by involving humans, in a pearl farm.