Pearls form when mollusks secrete layers of nacre (aragonite platelets bound by conchiolin) around an irritant. Natural pearls are extremely rare; most market pearls are cultured by inserting a bead or tissue nucleus into oysters or mussels.
Luster results from light interference in thin nacre layers. Value depends on size, shape (round preferred), surface quality (few blemishes), color, and overtone. X-radiography reveals nucleus type and nacre thickness for identification and quality assessment. Bleaching, dyeing, or coating may be applied and must be disclosed.