Travertine is a sedimentary rock. It is a natural chemical precipitate of carbonate minerals; typically Aragonite, but often recrystallized to or primarily Calcite. Basically calcium carbonate is deposited from the water of mineral springs or rivulets saturated with calcium bicarbonate. The spring water can either be hot, warm or cold. The amount of deposits may increase with the waters temperature or when biotic material accelerates the precipitation.

When carbon dioxide-rich water percolates through rocks in limestone areas, the water dissolves the limestone (typical karst process) and becomes saturated with it. When the environment the water runs through, changes significantly (ex. drop in pressure and/or change in temperature) this causes the water to release the carbon dioxide as gas, much like fizzy drinks. The calcium carbonate then recrystallizes; small debris, scrub and living biotic material like (moss, algae, Cyanobacteria) are encrusted. The biotic material may survive and continue growing on top. Rich deposits of aged, dried and hardened travertine have already been mined by the Romans. The rock typically remains quite porous with numerous cavities. When exceptionally porous it is known as "Calcareous Tuff" (German: Kalktuff). When pure and fine, travertine is white, but often is brown to yellow due to impurities (other than carbonate minerals).

Petrified "Calcareous Tuff" with "impurities", weatherproof in a 400 years old wall	Calcium-carbonate-encrusted, yet growing moss, early stage of petrified "Calcareous Tuff"
Travertine in relatively pure, fine property	Travertine terraces at Mammoth Hot Springs, Yellowstone National Park