tuff

Rock formed from consolidated volcanic ash — the compacted, ceite debris of explosive eruptions, sometimes welded by its own heat into dense, glass-rich stone. Tuff is the material of Cappadocia's fairy chimneys, the moai of Easter Island, and much of the building stone of ancient Rome. It is soft enough to carve when freshly exposed, hard enough to endure when weathered. The Bandelier cliff dwellings in New Mexico are carved into tuff. The word is easily confused with tufa, which is a different rock entirely.