Calcite of the oyster-like shell Placuna placenta proved to combine an uncommonly good transparency with high resistance to blows. These characteristics were shown to reside in the nanostructure of the calcite. The strength of this type of calcite derives from its property that at the atomic level the energy of a blow is distributed over ‘sibling’ craters which prevent the formation of a break as seen in ‘regular’ calcite. The next challenge is the synthetic imitation of the calcite of Placuna placenta.