A novel ultrahard carbon allotrope, body-centered tetragonal C6 (space group I-4m2) presenting mixed sp2/sp3 carbon hybridizations is proposed by crystal chemistry approach and studied for the ground state structure and stability (both dynamic and mechanical) within density functional theory (DFT). The structure characterized by C4 tetrahedra connected with C–C trigonal carbons shows close relationship with C6 'glitter' devised about 30 years ago. While 'glitter' has underlying net labeled 'tfi' according to RCSR nomenclature, novel C6 called 'neoglitter' is identified as 'tfa' underlying net. 'Neoglitter' is characterized by large bulk and shear moduli approaching those of diamond, and very high Vickers hardness (HV = 85 GPa). High phonons frequencies  ~ 40 THz, close to diamond, are observed and attributed to trigonal C-C pairs stretching. The electronic band structure points to metallic-like behavior assigned mainly to the itinerant role of trigonal carbon π electrons. The presence of both trigonal (sp2) and tetrahedral (sp3) carbon in 'neoglitter' is relevant to an extensive research domain involving nanodiamonds and various electronic applications.