A limestone formed by the precipitation of the calcium carbonate in spring water upon exposure to air. It has been found in large quantities in the Tiber valley near Rome and elsewhere in Italy. It varies in colour from pale buff to orange pink and has been frequently used for the buildings of Rome, notably the Colosseum and the colonnade of St Peter's. It has also been used for outdoor sculpture such as Bernini's Triton fountain, also in Rome. Recently it has been exported in great quantities from Italy on account of its popularity as an external cladding material in some forms of modern architecture.