The results show that 'glitter' C 6 has the highest hardness (83 GPa), which allows us to assign it to a family of ultra-hard materials . Hardness of tetragonal BCN (65 GPa), although lower than those of cubic BC 2 N and diamond-like BN-C solid solutions , nevertheless exceeds the hardness of single-crystal cubic boron nitride (62 GPa ), so this new phase, if synthesized, could be a quite promising material especially considering its metallic-like behavior. Apparently, tetragonal equiatomic BCN can be synthesized only at extreme conditions in a rather narrow pressure-temperature range .