A systematic experimental study of fractionation of carbon isotopes during diamond crystallization in model systems near the IW and CCO buffers helped to estimate the effective partition coefficients of carbon isotopes between diamond and crystallization medium. In the systems Fe(Ni,Co)–C, near the IW buffer, diamond is heavier than the solution of carbon in metal melt by 4.5%c at 5.5 GPa and 1400–1500 °C. In the system (Na2CO3CO2)–C, near the CCO buffer, diamond is lighter than the carbonate fluid by 2.6%c at 7.5 GPa and 1400–1700 °C. The values of fractionation are close but not equal to calculated equilibrium values and decrease as the rate of diamond crystallization increases. With regard to the low effectiveness of carbon isotope diffusion in diamond, the effective partition coefficients of carbon isotopes obtained during real diamond crystallization are the most informative for interpretation of data for natural diamonds. Based on the experimental results, we propose a scheme of the primary isotope specialization of diamonds. Isotopically heavy diamonds (δ13CVPDB of 0 to –5%c) crystallize in zones of metal melts (in the case of isotope depletion, δ13CVPDB decreases to –10%c or lower). Isotopically light diamonds (δ13CVPDB of –7 to –10%c) crystallize in more oxidized mantle zones. The interaction of different types of mantle matter with contrasting redox characteristics causes wide variations in the carbon isotope composition of diamond and in the composition of diamond-hosted inclusions.