The physical–mechanical behaviour of the weathered San Giacomo granite, Sardinia, has been investigated using a combination of physical–mechanical tests and microstructural analysis. Fieldwork and microscopy suggest a pervasive microfracture system developed on a pre-existing magmatic quartz anisotropy. Alteration indices, chemical analyses and mineralogy indicate that the yellow staining is produced by the circulation of oxygen-rich rainwater through microfractures in the rock, resulting in the oxidation of biotites. Apart from the obvious modification of the original grey colouring, the precipitation of Fe-oxides and hydroxides has welded fractures together, leading to the partial recovery of the physical–mechanical properties previously compromised due to microfracturing.