While travertine and limestone are both predominantly made up of calcium carbonate (calcite), their origins are different. Limestone tends to come from rocks formed in marine environments, while travertine comes from inland water sources, where layers of the rock form over time in different stages of growth, similar to tree rings.
Travertine often contains spherical pockmarks that are created by air bubbles that are trapped within the material, giving it a kind of ‘Swiss cheese’ look. The stone was particularly popular with the ancient Romans, who built structures such as the Colosseum and the Trevi Fountain from travertine.
The best travertine has a high density and is very tough, which is why it was used so extensively in Italian buildings. A cubic metre of good quality travertine can weigh 2.7 tonnes per cubic metre and has impressive flexural strength. Quarries are quite prolific in Europe, making travertine a popular choice of building material throughout the region.
Source: http://www.getty.edu/conservation/publications_resources/newsletters/11_2/news2_3.html