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If this somehow became possible and a known rare mineral fortune was found it would drop the commodity price of that mineral like a stone.
Not really, because whatever rare mineral they brought back would be controlled entirely by them, and they could demand whatever price they wanted for it. A bit like how OPEC controls the oil price.
No - surely the obvious thing is that it would cost so many billions of billions to mount such an operation, that it would never be commercially viable, no matter what they came back with and how much it was worth.