A mollusk is an invertebrate animal that is the source of all pearls. The mollusks that are important in jewelry are composed of mantle tissue (the body of the mollusk) and a bivalve hard outer shell. The mantle tissue is the most important part of the mollusk in pearl formation because it secretes the conchiolin that acts as a glue and the calcite and aragonite crystals, that together compose nacre, giving pearls their luster and orient.