We report lateral deformation experiments on rare earth substituted glasses from percalcic CaO-Al2O3-SiO2-Tm2O3. Using instrumented Berkovich indentation with normal loads below 100 mN, the scratch hardness was obtained from the work of lateral deformation per deformed volume in the elastic-plastic regime. The positive effect of rare-earth substitution on normal hardness, Poisson's ratio and Young's modulus was found to extent also to scratch hardness: in 25CaO-15Al2O3-60SiO2, partial replacement of alumina by Tm2O3 leads to an enhancement of the scratch hardness by about 1 GPa per 2 mol% of substitution, reaching the highest values of scratch hardness which have been reported for oxide glasses thus far. The enhancement of scratch hardness is attributed to enhanced cation packing fraction and super-structural cohesion as a result of rare earth substitution. The previously reported linear correlation between scratch hardness and bulk modulus is confirmed. © 2019 The Authors