The use of dark metamorphic stones in architecture is common through the centuries. Such materials were traditionally employed mainly for decorative purposes, both indoor and outdoor, and for architectural cladding. Unfortunately, after prolonged outdoor exposure their characteristic surface colour tends to fade. In the framework of the pilot conservation site of the Renaissance façade of the Cathedral of Monza (Italy), a thorough study of the features and state of conservation of an example of this lithotype, Oira stone, was conducted. This stone was employed during the 19th century restoration of the façade, and the on-site evaluation of the stone blocks after more than a century of exposure showed a distinctive chromatic alteration. The formation of a fragile superficial layer with scaling and detachments was also observed. Samples of the stone were studied using a multi-analytical approach to identify the deterioration mechanisms involved. The results showed that the colour variation is associated with the chemical alteration of the stone, resulting in the selective leaching of magnesium ions from the phyllosilicate structure of the most external material. At the same time, a surface recrystallization of mainly low-ordered SiO2 occurred. The role of chemical leaching by atmospheric interaction in stone discoloration was also investigated by laboratory accelerated ageing of serpentinite specimens.