The purpose of this investigation was to extend the 'graphite-to-diamond' G2D-like changes of carbon crystal systems implying 2D C(sp 2 ) plane paving to 3D C(sp 3 ) tetrahedral, to 3D trigonal  3D tetrahedral. Based on crystal chemistry rationale and DFT calculations of ground state energies and the derived physical properties, an original body centered tetragonal carbon allotrope C 20 with dia (diamond) topology was devised and demonstrated to behave like diamond for all the physical properties, whence its qualification as pseudodiamond.