Travertine

Travertine is a terrestrial sedimentary rock. It is a natural chemical precipitate of carbonate minerals; aragonite or calcite. This happens from solution in ground and surface waters, and/or geothermally heated hot-springs.^[1][2] Similar (but extremely porous) deposits formed from ambient-temperature water are known as tufa.

Travertine terraces at Mammoth Hot Springs, Yellowstone National Park

Calcium-carbonate-encrusted, yet growing moss, early stage of porous travertine formation.

The Colosseum built by the Romans, was made of travertine.