The results are summarized in Tables and with relevant references to the literature data. From Table the density of novel tet-C 4 is found within the range for hexagonal (lonsdaleite) and cubic diamond. This also stands for Vickers hardness and bulk modulus (B 0 ) calculated in the framework of the thermodynamic model of hardness. Table presents the hardness calculated using all four theoretical models, and other mechanical properties such as shear modulus (G), Young's modulus (E), the Poisson's ratio () and fracture toughness (K Ic ). Presently proposed tet-C 4 exhibits hardness and fracture toughness magnitudes very close to those of diamond (both cubic and hexagonal). It should be also noted that hardness of tet-C 4 is much higher than H V -values reported for all recently predicted carbon allotropes . A good agreement can be noted.