This paper describes the main results of an experimental investigation into habitual failure planes in pure Mg. The investigation involved testing a set of pure Mg single crystal specimens in tension and compression as well as a set of pure Mg oligocrystal and pure Mg polycrystalline specimens in tension. The microstructural characterization was performed via electron backscatter diffraction, while fracture surfaces were observed with confocal laser-scanning microscopy and X-ray micro–computed tomography. Several failure planes observed in prior literature were confirmed in the present investigation. Nevertheless, another plane was frequently observed as a cleavage plane, which is neither a slip plane nor a twin plane in Mg. Furthermore, the minimum necessary surface energy for rupture was calculated for all possible planes and underlying atomic bond densities based on interatomic potential data for pure Mg. Evaluating the relationships between planar bond density and habitual fracture planes revealed that the cleavage planes strongly favor the orientations that are predicted to necessitate the lowest surface energy. These relationships as well as the new observations are presented and discussed.