A number of years ago, when I was an undergraduate student, I had the privilege of going with my adviser on many trips in the field. I learned more on those trips than I think I did in the classroom. On one trip he took me to several hot springs in the Black Rock desert of central Utah, and I got to observe the deposition of travertine first-hand.

Black Rock travertine

Travertine is a carbonate deposit most often associated with hot springs. As the water travels through the Earth and is heated by some deep, magma source, and then circulates back to the surface, it dissolves a host of minerals. On the surface, through either evaporation or the action of microbes the minerals precipitate out of the water, slowly building up layer upon layer of stone. The vesicles and contaminates of the travertine give is unique and interesting texture, and thus make it a popular decorative stone for tiles.
Several years prior to my visit with my professor, he was there with another student and they studied the travertine and hot springs of the region, and they found the remains of a fossil bird, an American Avocet, preserved in the stone. It seems that the bird had died in one of the hot springs, and the travertine formed around the bird’s skeleton, preserving impressions of feathers as well as the bones of the body.

American Avocet skeleton preserved in travertine

In addition to the bird, remains of insects in the travertine were common. These fossils give new meaning to the term set in stone.

For other stories on travertine, see:
An introduction to travertine tiles
Advice on installing travertine tiles