I would strongly suggest that you do NOT invest in things that are underground, be it Gold, Copper, Diamond, Oil, or Magnesium (LOL).
(with regards to Magnesium, you may refer to my discussion under the "JES" stock thread, and the "Inno-Pac" stock thread for Diamond, at the TPG Forum)
What you can't see, you can't know and you should NOT invest in them.
Just a personal opinion and a personal choice.