"is diamond the hardest material" as generally admitted? In this context, hexagonal 6H carbon (P6 3 /mmc space group) was claimed to be harder than diamond , and we recently discussed the ultra-hardness of a novel rhombohedral carbon allotrope (rh-C 4 ) which can be considered as an extended hexagonal h-C 12 lattice presenting mechanical properties like those of diamond (Vickers hardness H V ~ 100 GPa).