Design of hypothetical crystal networks, consisting of most pentagon rings and generically called diamond D 5 , is presented. It is shown that the seed and repeat-units, as hydrogenated species, show good stability, compared with that of C 60 fullerene, as calculated at DFT levels of theory. The topology of the network is described in terms of the net parameters and Omega polynomial. IntroductionThe nano-era, a period starting since 1985 with the discovery of C 60 , is dominated by the carbon allotropes, studied for applications in nano-technology. Among the carbon structures, fullerenes (zero-dimensional), nanotubes (one dimensional), graphene (two dimensional), diamond and spongy nanostructures (three dimensional) were the most studied (Diudea 2005;Diudea and Nagy 2007). Inorganic compounds also attracted the attention of scientists. Recent articles in crystallography promoted the idea of topological description and classification of crystal structures (Blatov et al. 2004(Blatov et al. , 2009Delgado-Friedrichs and O'Keeffe 2005).Diamond D 6 (Fig. 11.1), the classical, beautiful and useful diamond has kept its leading interest among the carbon allotropes, even as the newer "nano" varieties. Along with electronic properties, the mechanical characteristics appear of great