It was recalled that diamond was mechanically exceptional in that it resisted volume changes (exceptional bulk modulus, B) more markedly than did any other known substance, resisted shear stress more than did any other substance, had an octahedral shear modulus which was 14% greater than B and was harder than anything else. Simple quantitative theories of the bulk modulus, the shear modulus, and the stress required to move dislocations in diamond were developed here and were compared with experimental measurements.

Why Diamond is Very Hard. J.J.Gilman: Philosophical Magazine A, 2002, 82[10], 1811-20