Diamond crystals 0.05-0.2 mm (rarely up to 0.7 mm) in size were found in lavas of the Tolbachik fissure eruption (TFE, Kamchatka, 2012-2013). The TFE basaltic lavas with diamonds in the pores are similar in chemical composition to the lavas of the Great Tolbachik Fissure Eruption (GTFE, Kamchatka, 1975-1976); however, the basalts of the new eruption contain slightly more SiO2 and higher Fe relative to those of GTFE. Basalts of both eruptions are close in trace element composition. The diamonds were identified using electron microscopy and X-ray diffraction. The crystals, as a rule, represent a combination of cubes and octahedrons, with rare rhombododecahedrons. The TFE diamond has a typical cubic cell with a cube edge a = 3.574 Å and belongs to the spatial group of Fd-3m. The X-ray diffraction pattern of some grains contain reflexes with fractional indices, which indicate the twinning of the diamond along the plane {111}, which is confirmed by crystal morphological analysis. The TFE diamond was formed at the early stage of eruption with typical intense gaseous-pyroclastic jets to a height of 250 m and more. The finding of diamonds in fresh products of basaltic eruption of an active volcano may be considered as evidence of its magmatic source. Such numerous (hundreds) diamond grains in lavas of the active volcano were found for the first time in the world.