[en] A direct method of determination of the valence-electron structure from its crystal structure has been presented by the writer at XIIth International Congress of Crystallography, at Ottawa, 1981. Here the method is applied to determine the valence-electron structures of Y₃Al₅O₁₂ (YAG) and Al₂O₃, Cr₂O₃ (ruby) to see how certain properties of these laser materials are related to their valence-electron structures. The first point observed is the very strong, continuous, but zig-zag three-dimensional fractional covalent M-O bond nets (M = Al or Cr) being connected with the high melting points, the great resistance against the puncture by the strong laser beam. In the case of Nd-YAG, the distortion caused by the replacement of the Y atoms by almost the same size Nd atoms is small. On the other hand, because of the similarity of the valence-electron structures of α-Al₂O₃ and Cr₂O₃, the replacement of Al atoms by the little larger size Cr atoms in small amount is easily understood. The large atomic spins of magnetic moment m_B = 2.76 μ_B (= experiment value, the theoretical m_B = 2.70₂ μ_B) of Cr atoms pointing parallel to the c axis of the hexagonal lattice, is found to be due to the direction of the spin situated in the space of lowest density space of the valence-electron cloud distribution. This agrees with the result of α-Fe disclosed in the paper Electron theory of the magnetic moment structures of α-Fe, epsilon-Co, Ni from neutron diffraction experiments presented at the Symposium on Neutron Scattering, Argonne National Laboratory, 1981. This will be found also true in many other oxides like α-Fe₂O₃, FeTiO₃, MnTiO₃ and so on