Conchiolin

From Freepedia

Conchiolin (sometimes referred to as conchin) and perlucin are complex proteins which are secreted by a mollusc's outer epithelium (the mantle) to form chambers which hold and bond to the crystals of aragonite which give such shells their stiffness. The ions necessary to form calcium carbonate are also secreted by the mantle, but it is the tailored ionic environment of the perlucin protein which causes aragonite (rather than calcite) crystals to nucleate [1], in much the same way that collagen nucleates hydroxyapatite crystals. Conchiolin serves as a relatively flexible, crack-deflecting matrix for the mineral aggregate particles; its strength and the strong bonding of perlucin give the finished material (nacre) an impressive level of toughness.