The stone used for the bell towers on the Nativity and Passion facades is sandstone from Montjuïc in Barcelona. Due to the scarcity of this exact type of stone (the Montjuïc quarries were closed several years ago and the stone is only obtainable from demolished buildings), other sandstones and granite have been used for the windows and parts of the towers and roofs. Reinforced concrete, which Gaudí used for the pinnacles on the Nativity facade, has been used in accordance with his instructions on the construction of the naves.