We compare high resolution roughness measurements on fractured granite,
basalt and sandstone samples. For granite and basalt, the fractures display over the full
experimental length scale range a self-affine geometry with a characteristic Hurst
exponent consistent with the value 0.8 suggested to be universal by several authors. For
sandstone, the rugosity spectrum reMayns independent on the fracturation velocity but
displays self-affine characteristics only for characteristic length scales roughly larger
than the grain size: furthermore, the corresponding Hurst exponent takes a value
0.47 ± 0.05 distinctly smaller than the previous one. At characteristic length scales below
the grain size, a clear cutoff is observed in the spectrum. This cut-off is shown through
numerical simulations to be largely explainable by the faceted shape of the sand grains
and to a minor degree by the finite size of the profilometer sensor tip. None of these
factors seems to influence the low frequency part of the spectrum from which Hurst's
exponent is determined. The differences between the values of Hurst's exponent may
reflect the intergranular nature of fracture in the sandstone samples while no effect of
structural heterogeneities on fracture propagation is observed in granite.