The hardness and mechanical properties of qtz C 6 are, as expected, close to those of diamond and lonsdaleite. The corresponding values for qtz BN and especially for qtz SiC are significantly lower and are at the level of the mechanical properties of cubic boron nitride and cubic silicon carbide, respectively. It should be noted, however, that the hardness of all three phases with quartz topology is about 5% higher than that of the corresponding cubic phases. Such increased hardness is probably related to the ultrahigh densities of the qtz phases resulting from the distorted tetrahedron building blocks.