One of the two main types of stone used in ancient Roman buildings. It was cheap and easily worked, but too soft to use in load-bearing structures or places subject to heavy wear (Travertine was used in those areas).
Roman buildings were made of concrete, so a soft stone such as tufa could be safely used for cladding, giving the appearance of stone building at lower cost.
Augustus Caesar was fond of boasting "I found Rome built of brick, and left it built of marble." It would be more accurate to say that he left it built of tufa cladding.