As the hardest substance known, diamond is ideal for cutting rock and other tough stuff. But diamond is costly, and it degrades when machining steel and other ferrous metals because of reactions that create softer iron carbides. For cutting steel, the first choice is cubic boron nitride, which is almost as hard, resisting 40 to 50 gigapascals (GPa) of pressure compared with diamond’s 70 to 100 GPa. But manufacturing the substance requires high temperatures (1,500 degrees Celsius) and extreme pressures (5 GPa), which make it expensive.