Which Substance Is the Hardest?

Diamond is pure carbon arranged in a tetrahedral crystal lattice where each atom is covalently bonded to four neighbours — the strongest arrangement of atoms in nature. On the Mohs scale (1-10, qualitative), diamond is 10; on the Vickers hardness scale (quantitative), natural diamond measures 70-150 GPa depending on orientation. Synthetic nanocrystalline diamond (aggregated diamond nanorods, or ADNR) is even harder than natural single-crystal diamond because its randomly oriented grains prevent crack propagation along crystal planes. Lonsdaleite (hexagonal diamond, found in meteorites) is theoretically 58% harder than diamond, though large samples have not been produced for direct measurement. Wurtzite boron nitride (w-BN) is calculated to be harder than diamond under indentation but is too rare to test at scale. In 2022, researchers created AM-III (a fullerene-based carbon material) with a Vickers hardness of 113 GPa, rivalling diamond. Applications for super-hard materials include cutting tools, drill bits, abrasives, and protective coatings.