Travertine is a natural stone deposited by geothermally heated hot springs. It is considered to be a type of limestone which forms due to rapid precipitation of calcium carbonate, hence, it is found around mineral spring deposits.
If we are being technical and extremely specific then we can refer to travertine by its mineral name which is calcium carbonate. It is formed by the precipitation of carbonate minerals and its porosity is due to the presence of organisms such as macrophytes, bryophytes, algae, etc. Its natural colour is white but travertine is also found in tan, creamy or rusty varieties. The red colour of travertine and its different intensity is due to the presence of iron carbonate.
When minerals dissolve in the groundwater and get moved by natural springs and rise above the ground, travertine is created. It forms large blocks, is taken from quarries and cut down into smaller blocks for more practical loading and unloading processes.
About 30 years ago, until the 1980s, Italy was the main supplier of travertine and had a monopoly on the market. The most famous quarries in Italy are in Guidonia Montecelio and in Tivoli, while the name of the latter district and the word travertine have the same origin from the Latin word tiburtinus through the Italian travertino.
The natural stone was quarried in Italy since Ancient Roman times and was used primarily as building material. Much of the structure of The Colosseum in Rome was made from Travertine.
Nowadays, significant travertine suppliers are Turkey, Mexico, Peru and Iran. If you've ever visited the ruins of Hierapolis (photo below) in the Turkish province of Denizli, you might already know that this ancient city was built almost entirely with travertine. Many Roman settlements in Turkey were built using this stone, with engineers making frequent use of travertine as a common building material.
(By Contributor ITA - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=33311399)