Abstract <p>The role of the methane–hydrogen fluid is one of unresolved questions of diamond genesis under reduced mantle conditions. This work presents the results of experimental studies of the effect of various anthracene (С₁₄Н₁₀) contents on the crystallization, properties, and characteristics of diamond in metal–carbon melts at <i>PT</i>-parameters of lithospheric mantle. It is found that an increase in the anthracene content in the Ni₇Fe₃–С system from 0.00 to 2.69 wt % at a pressure of 5 GPa and a temperature of 1400°С leads to a decrease in the degree of the graphite-to-diamond transformation from 100% to zero indicating the inhibitor role of anthracene. Metal, graphite, methane, and hydrogen are the main inclusions in synthesized diamonds. It is substantiated that the fluid composition of the studied system is methane–hydrogen and it is considered the inhibitor impurity. It is established that during the increase in the anthracene content of the system, the diamond growth on seeds is replaced by spontaneous crystallization, further formation of metastable graphite and diamonds with elements of antiskeletal growth, and further crystallization of metastable graphite only.</p>