The extreme hardness of diamond is due to the three-dimensional (3D) arrangement of carbon atoms within corner sharing C4 tetrahedra with d(C-C) = 1.55 Å equal to the sum of two r(C) = 0.77 Å atomic radii. The resulting 3D system is a perfectly covalent network. Such tetrahedral arrangements are also found in 3D BN and carbon nitrides as well as novel C 6 N 8 discussed herein.