Tuff is a soft rock formed of compacted volcanic ash, sometimes including pumice and scoria. It tends to be porous is nature and can be fragile. Though far from ideal for detailed carving tuff has been used for building at Rome throughout the city's history. Thick beds of tuff surround Rome and though easy to quarry tuff hardens when exposed to air making it a useful building stone. Tuff can also be cut using a toothed saw and quickly shaped into ashlar blocks or the small pyramidal blocks used in opus reticulatum.