The only substances that might have a chance to be profitably mined from asteroids are those that on Earth are concentrated at depths too great to be accessible and they are depleted at the Earth surface by thousands of times, or at least hundreds of times.
These elements are:
1. The 6 platinum-group elements
2. Rhenium and gold
3. Tellurium, selenium and germanium
Besides these elements, only indium might be profitable.
Indium is depleted only a little at the Earth surface, but its original abundance is very low, so additional extra-terrestrial sources could be useful.
From all the rare elements, indium is the one for which there are no good substitutes in its main applications.
Indium is completely irreplaceable in the best lighting sources (i.e. LEDs), in the best power semiconductor devices (with gallium nitride) and in the best transparent conductors (required e.g. for computer displays).
For transparent conductors there are great efforts to find a substitute and those might eventually be successful. Nevertheless until now all the alternatives have various disadvantages.
