The building stone of medieval Prague is white or yellow tinted opuka. Its hegemony is occasionally disrupted by non-traditional stone material, mainly various kinds of sandstone or black-grey Ordovician slate.
Sporadic constructions from this dark atypical material raised great interest in the early days of detailed research of Prague historic centre. The knowledge of the buildings with massive use of slate was revised after several decades of intensive rescue excavations.
Comprehensive catalogue proves the use of Ordovician slate as construction material in the medieval Prague as absolute exception. The traditional presumption that the slate houses are contemporary with the later stage of the Romanesque ashlar houses was not confirmed.