For the first time, the detailed structure of χ-Al2O3 and η-Al2O3 surface has been established by implementing the NMR crystallography approach. The surface of η-Al2O3 has been demonstrated to be formed primarily by the (111) facets, while the χ-Al2O3 surface is a combination of (111) and (110) facets. This observation supports the block model of aluminum oxides previously proposed by Tsybulya and Kryukova [S. V. Tsybulya, G. N. Kryukova, Phys. Rev. B 77 (2008) 024112.]. The additional terminal OH groups, observed experimentally and not contributing to (111) and (110) theoretical calculations, are considered to be bonded to the tetrahedral aluminum sites. Their origin is related to the junctions of crystallographic faces of spinel building blocks, being a part of discussed model. Higher content of these terminal OH groups in χ-Al2O3 is a result of more junctions in the case of its more mosaic structure compared to η-Al2O3.