Based on recent experiments, the diborides OsB 2 and ReB 2 were proposed to be ultraincompressible and superhard materials. By application of an ab initio density-functional theory approach we investigate the elastic and cleavage fracture properties of the borides MB 2 ͑M = Hf, Ta, W, Re, Os, and Ir͒. We derive a direct correlation between the lowest calculated critical cleavage stress and the experimental ͑micro͒hardness. By calculating the critical shear stress and estimating the possibility

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... f dislocation emission we can justify the prediction that ReB 2 is indeed a superhard material.