FYI- The previous post on marble is a direct copy from Wikipedia. Credit where credit is due!
Here is a link to an excellent article on the formation and uses of Limestone from Geology.com
One of my favorite bits of trivia: when you open up the foil on a traditional stick of chewing gum, guess what that little bit of white powder is? Calcium carbonate, often in the form of limestone or marble dust. Some stones can be ingested! It is often used as a desiccant or a filler in foodstuffs, toothpaste, or as an inert filler
for tablets and other pharmaceuticals.
It is designated as E170 as a food additive. Ingesting too much can be hazardous, however, so don't start throwing bits of Jerusalem Gold in the blender with your morning smoothy.