The BBC put together a world resource list a couple years back that I think you might find interesting. http://www.bbc.com/future/story/20120618-global-resourc

The BBC put together a world resource list a couple years back that I think you might find interesting. http://www.bbc.com/future/story/20120618-global-resources-stock-check 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.