For those metals, cubic boron nitride works and has about half the Knoop hardness of diamond (and more than twice the hardness of aluminum oxide.) c-BN is isoe

For those metals, cubic boron nitride works and has about half the Knoop hardness of diamond (and more than twice the hardness of aluminum oxide.) c-BN is isoelectronic with diamond; h-BN is isoelectronic with graphite (but is an insulator).