Travertine is a form of limestone, and is related to marble, actually falling somewhere between marble and limestone in terms of its development over thousands of years, deposited by mineral springs, especially hot springs, underground rivers and other water sources.
Travertine tile is very vulnerable to acids, which can make it unsuitable for certain installations. Kitchens are a poor choice, as orange juice and other acidic foods may be spilled on the tile, potentially damaging it. Travertine tile should be well sealed to protect it from moisture, spilled items, and wear and tear, and it can be a good idea to periodically strip and reseal the tile to keep it in good condition.