An accurate, X‐ray diffraction study is made of the atomic and electronic structure of the ruby crystal Al2O3:Cr3+. Least‐squares high‐angle refinement (R = 0.0069, Rw = 0.0099, S = 1.055) for 180 independent reflections shows that the Cr‐atoms substitute the Al‐atoms not precisely at the sites of the latter, but are shifted along the c‐axis at a distance of 0.13 Å towards the vacant octahedral holes; this is in agreement with some previous spectral data. In the electron deformation density map this displacement manifests itself as a peak 0.30 Å−3 which is absent in the pure rorundum crystal. The deformation electron density and the electrostatic potential distribution in cuby crystal are analyzed in detail.

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