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Nanoscale structural and mechanical characterization of the shell of a red abalone has been carried out. Cobble-like polygonal nanograins are basic building blocks that are used to construct individual aragonite platelets into a mother-of-pearl configuration known as nacre. The nanograin-structured aragonite platelets are not brittle in nature, but somewhat ductile. The deformability of the aragonite platelets together with the crack deflection, aragonite platelet slip, and organic adhesive interlayer contribute to the nacre's fracture toughness. Cracks formed in the outer prismatic layer of the shell do not show the crack diversion mechanism.