Formed when layers of calcium carbonate form around a piece of foreign matter within the shell of a mollusk. Though pearls can form in a variety of shapes, the most popular for jewellery are spherical, where the iridescent layers have formed in concentric layers. Naturally formed pearls are very rare, though there are many cultured pearls on the market. See also – akoya pearls, baroque pearls, cultured freshwater pearls, round pearls, south sea pearls, symmetricals.