is the pearly substance secreted by the mantle of certain molluscs to form a pearl Nacre also creates the beautiful mother-of-pearl coating found on the inside of pearl shells and several other varieties of shellfish
The calcium carbonate-based substance secreted by a pearl in response to an irritant, such as a piece of shell (cultured pearl) or parasite (most natural pearls)
having a play of lustrous rainbow-like colors; "an iridescent oil slick"; "nacreous (or pearlescent) clouds looking like mother-of-pearl"; "a milky opalescent (or opaline) luster"