Diamond has the highest bond energy per unit volume of all known materials, and hence it is assumed to possess the highest hardness. Diamond’s hardness comes from its small atoms that each of them forms four covalent bonds. To make a structure harder than diamond, its atoms must be smaller than carbon, and/or these atoms form at least four covalent bonds. The first consideration would rule out all elements with period number higher than 2. The second criterion would eliminate all elements lighter than carbon. Hence, only C, N, O, F, and Ne are possible candidates of superdiamond. However, in order to beat diamond in hardness, these elements must form mono-atomic structures with coordination number higher than 4. Moreover, no lone pair electrons are allowed, so all of their valence electrons must be used to form single covalent bonds.