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.

References
  1. Dictionary of geological terms, 1962. A Dolphin Reference Book
  2. A glossary of karst terminology, 1970. Geol. Surv. Water-Supply Paper 1899-K, U. S. Gov. Print. Off., Washington.
Travertine in a 400 year old wall of the castle Hohen Tübingen. The material is extremely hard and weatherproof. The cavities and impurities - remnents of small debris, scrub and biotic materials - make the rock porous and lightweight.
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