Travertine is a type of limestone often found in the mouth of hot spring or in a limestone cave. Its characteristic feature is its irregular shaped pitted holes throughout its surface. In architecture, it is used as a building material, to cover other materials and in outdoor sculpture. 
Travertine is quarried in the Tiber Valley near Rome. As a building material, Romans used it for building temples, aqueducts, monuments, bath complexes, and amphitheaters such as the Colosseum.