Nitrogen is by far the most abundant element in the Earth's atmosphere but most of it stays aloof in a form which living things can't use.

BBC: Leaving our mark: Fossils of the future

The most compelling case for asteroid mining is for a yet-undesired element that turns out to be not abundant in the earth's crust and becomes suddenly valuable because of some future technological advance.

WSJ: Exhausting the Earth's Resources? Not So Fast