During a wider study of the correlation between the purity and the physical properties of β rhombohedral boron, Knoop hardness and room resistivity measurements were performed on samples prepared by different methods (chemical vapour deposition (CVD) and fusion) and precisely analysed with a spark source mass spectrometer. We systematically measured the hardness (taking into account the surface state, the load, the anisotropy and the grain size) to determine small hardness variations within the range of 10%. Hardness HK200 (kg mm−2) increases regularly with the impurity content C (ppm) according to the equation HK200 = 0.015 λC + 2320 where λ = 1 for the molten samples and λ = 6.2 for the CVD samples; the different slopes are probably due to a different impurity distribution: the logarithm of the room resistivity ρ (MΩ cm) increases linearly with the purity according to the equation log10ρ = (0.6 −1.1) × 10−4C for samples of grain size 5-500μm; the Knoop hardness and resistivity are anisotropic in single crystals.