Origin of ultrahardness of 6H diamond: harder than cubic diamond

Combining first-principles calculations and quantitative predictions of a semiempirical hardness theory developed recently, we show that 6H diamond (6H-C), a representative of noncubic polymorphs of carbon can be harder than a cubic diamond. Such eminently high intrinsic hardness exceeding that of diamond is identified theoretically for the first time. The results challenge the conventional wisdom that cubic diamond is the hardest single crystal.