Shells are composite materials of calcium carbonate (found either as calcite or aragonite) and organic macromolecules (mainly proteins and polysaccharides). Shells can have numerous ultrastructural motifs, the most common being crossed-lamellar (aragonite), prismatic (aragonite or calcite), homogeneous (aragonite), foliated (aragonite) and nacre (aragonite). Although not the most common, nacre is the most studied type of layer.