The numerical values are given in the last column of Table . H V (C 4 ) = 97 GPa is a magnitude in agreement with the admitted values of diamond and H V (C 5 ) = 81 GPa let consider it as ultra-hard . Presently investigated C 6 is found characterized with a smaller albeit large hardness. Finally, with H V = 15 GPa, C 3 is described as a soft material.