Tuff

Tuff (from the Italian tufo) is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is compacted into a solid rock in a process called consolidation. Tuff is sometimes erroneously called “tufa”, particularly when used as construction material, but properly speaking tufa is a limestone precipitated from groundwater. Rock that contains greater than 50% tuff is considered tuffaceous.
Tuff is a relatively soft rock, so it has been used for construction since ancient times. Since it is common in Italy the Romans used it often for construction. The Rapa Nui people used it to make most of the moai statues in Easter Island.
Tuff can be classified as either sedimentary or igneous rocks. They are usually studied in the context of igneous petrology, although they are sometimes described using sedimentological terms.