Travertine tiles are formed from hardened mineral deposits that are left over after carbon dioxide-rich water filters through rocks and earth. As the water passes through areas with concentrations of limestone, the limestone dissolves. When this limestone saturated water reaches the surface, the carbon dioxide is released due to the changes in temperature and pressure. As the carbon dioxide fizzes out of the water, the limestone is left behind, and re-crystallizes over underwater plants. As these mineral deposits harden into stone, they can be made into tiles. Travertine tiles are characterized by fissures and small irregular holes on the surface of the tile that are not found in limestone.