Travertine is a limestone sedimentary rock, often used as building material, particulary in Rome, since I millenium a.C.
The difference between soft (or pourus) limestone deposits and banks of travertine is essentially due to the geological conformation of the formation ground: limestone deposits are so frequent in nature being formed by the precipitation of carbonate minerals from solution in ground and surface waters.
Travertine formation is helped by sub-aerial continental environment where limestone solution had enough time to stagnate and settle in a flat terrain, near enough to the surface to cycle emersion and submersion, not disturbed by hot springs or freshwaters.
The colour of travertine depends on the oxides he has incorporated.
Natural colouring varies from white, tan to cream-coloured, across various shade from yellow to red.
Industrial quality of the sedimentary bank depends essentially on his compaction. Generally travertine still remain a strong and docile stone, usefull for flooring and lining, both internal and external, and also, in some cases, for sculpture.