In this work, we have studied a specific series of individual diamonds and their aggregates from Zarnitsa kimberlite pipe and diamonds of variety V-VII from the placers of the northeast of the Siberian Platform which are representing the dark gray- to black-coloured crystals of transitional octahedron and rhombic dodecahedron habits. Internal structures in the Zarnitsa kimberlite pipe diamonds are derived from the crystallization of the polycrystalline cores, according to the geometric selection rule. Unlike the mosaic block structure of similar diamonds, described in the placers of the North-East of the Siberian platform, which were formed by the splitting crystal growth mechanism. Thus, despite the physical resemblance and the similarity of the internal structure, diamond crystals from the Zarnitsa kimberlite pipe and the rounded diamonds from the alluvial placers were formed due to essentially different processes. The carried out researches have shown that the differences of these two groups of crystals are reflected not only in the mechanisms of internal structure formation, but also in the paragenesis of inclusions, defect-impurity composition and isotopic composition of carbon.