Mother of pearl, or nacre, is one of a number of characteristic skeletal structures of molluscs, occurring in cephalopods, gastropods and bivalves. It consists of plates of aragonite, about 0.3 μm thick, arranged in sheets, with a tenuous protein matrix. Mechanical tests of nacre from all three classes show that it has a tensile strength of between 35 and 110 MN m^-2. It is slightly viscoelastic, and shows marked, though not extensive, plastic deformation. The maximum measured strain was 0.018. While under­going plastic deformation the material shows considerable optical changes. The regions where plastic flow is occurring show ‘tension lines’, probably equivalent to similar lines in bone. These are probably caused by voids forming in the protein matrix. The work of fracture is very different in different loading directions, being about 1.65 x 103 J m^-2 when fractured across the grain, and 1.5 x 102 J m^-2 when fractured along it. Nacre shows considerable ability to stop cracks.

An attempt is made to explain qualitatively the mechanical behaviour of nacre in terms of its submicroscopic structure. It is concluded that the precise geometric arrangement of the plates is most important, and that this constraint may make nacre less suitable for shells that must be built quickly.