Travertine, consisting of calcium carbonate, forms around hot springs and in caves. It has a porous structure like a sponge. These pores are caused by the gases that exist within it during its natural formation and then escape. It is generally known as a type of marble, but it differs from marble with its porous structure. It is a building material that has been frequently used since ancient times. Travertines have not gone through as long a formation process as marble and granites. It came to the surface before its formation was fully completed.