The 35 paired diamond intergrowths of rounded colorless transparent and gray opaque crystals from the placers of northeastern Siberian Platform were investigated. Mineral inclusions (KFsp, Coe, E-Grt, Po) detected in studied samples belong to eclogitic paragenesis. The majority of studied samples have uniform ranges of nitrogen content (1126–1982 at. ppm) and carbon isotope composition (− 16.8 to − 23.2 ‰). These characteristics pointing towards subducted material are possible sources for their genesis. Two samples consist of a gray opaque crystal with the subduction-related characteristics (δ13C ca. − 21‰ and N ca. 1300 at. ppm) and a transparent crystal with low nitrogen content (412 and 29 at. ppm) and a heavy carbon isotopic composition (δ13C − 4.2 and − 4.6‰) common for primary mantle range. The higher degree of nitrogen aggregation in the crystals with mantle-like characteristics testifies their longer storage in the mantle conditions. These samples reflect multistage diamond growth history and directly indicate the mixing of mantle and subduction carbon sources at the basement of subcontinental lithospheric mantle of northeastern Siberian Platform.