Travertine is a type of rock which is often used as building material in Rome. It is a natural chemical precipitate of carbonate minerals; typically aragonite, but often recrystallized to or primary calcite; which is deposited from the water of mineral springs (especially hot springs) or streams saturated with calcium carbonate. When pure, travertine is white, but often is brown to yellow due to impurities. In its pure form, it can be a cheap but good substitute for marble.
When carbon dioxide-rich water percolates through rocks in limestone areas, the water dissolves the limestone and becomes saturated with it. When the water resurfaces later, the sudden drop in pressure and the change in temperature cause the water to release the carbon dioxide gas, much like fizzy drinks. The calcium carbonate then recrystallizes, often over minute underwater plants. The resulting rock is typically quite porous with numerous cavities. When exceptionally porous it is known as calcarious tufa.
Extensive deposits exist at Tivoli, near Rome, explaining the extensive use of it in the city. In fact, travertine derives its name from this town. Tivoli was known as Tibur in ancient Roman times. The ancient name for the stone was lapis tiburtinus. meaning "tibur stone", which has been corrupted to travertine.