A pearl is a firm object formed within the soft tissue (specifically the mantle) of a living shelled mollusc.
Just like the shell of a clam, a pearl is collected of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The perfect pearl is dreamily round and smooth, but many other shapes (baroque pearls) occur.
The premium class natural pearls have been highly respected as gemstones and objects of attractiveness for many centuries. Since of this, pearl has turn out to be a metaphor for something rare, fine, admirable and appreciated.