Nacre crystal layers. Coloured scanning electron micrograph (SEM) of the layered crystal plates forming the nacre (mother-of-pearl) in a nautilus shell. The interior shell layer in a nautilus consists of this composite material, which is mostly a form of aragonite, a calcium carbonate mineral. Nacre has a very high tensile strength due to a combination of soft organic layers and hard inorganic platelets of aragonite. The nautilus is a marine cephalopod mollusc. Magnification: x3300 when printed at 10 centimetres high.