where ρ is a density and N is a coordination number. Considering that rs-ZnO is denser than w-ZnO, but has a higher coordination number (6 vs. 4), the hardness of rs-ZnO should be only 82% of that of w-ZnO, i.e. 3.9 GPa. The observed 5.5 and 7 GPa values can only be caused by the nanostructuring. The dramatic hardness increase in the case of nanostructured bulk rs-ZnO synthesized from commercial microcrystalline w-ZnO can thus be clearly attributed to the nano-size of the grains (Hall-Petch effect ) and the purity of the grain boundaries. A similar increase in hardness compared to single-crystal value has been observed in the case of nanocrystalline cBN obtained from bulk pyrolytic BN (+90%), nanostructured diamond (+70%) , and nano-stishovite (+50%). 7