Diamond is the hardest known material with a Vickers hardness (H V ) at the level of 100 GPa and density ρ = 3.635 g/cm 3 . Its structure (space group Fm-3m) is formed by corner-sharing tetrahedra of sp 3 -hybridized carbon atoms with C-C-C = 109.47° and is characterized by the highest atomic density (i.e., the number of atoms per unit cell volume) and the highest density per valence electron . A rare hexagonal form of diamond (lonsdaleite) (space group P6 3 /mmc) with virtually the same density has been claimed to be stronger and stiffer than diamond .