Diamond and cubic boron nitride (CBN) are considered to be the hardest known grinding materials and are therefore often referred to as "super-abrasives". For this reason they are suitable for machining materials which are difficult or even impossible to grind with conventional abrasives such as silicon carbide or corundum.
Diamond and CBN have the same crystal structure, with diamond consisting of pure carbon, whilst CBN is made up of the elements boron and nitrogen.
