So I know limestone is technically a non-renewable resource, since we're using it far faster than it is being created.
Should, uh, we actually be worried? It's like 7% of the Earth's crust; are we actually using limestone fast enough to use up a significant fraction of the usable limestone in the expected lifetime of humanity?
http://www.bbc.com/future/story/20120618-global-resources-st...
It lists a variety of elements (like antimony) that are surprisingly constrained. If I were to pick the most critical material on the list, though, it would be phosphates. The green boom that feeds the world needs phosphates, the world supply of phosphate-bearing rock is small, and there are no possible substitutes.