Why not just mine it on Earth; actually?. If there’s something which isn’t lacking on our planet, it’s most certainly iron.
Regarding Arthur C. Clarke novella, I certainly wouldn’t throw away an hypothetical Lunar diamond. Even though we’re already able to produce affordable synthetic diamonds, natural diamonds from Earth are still sold at aburdly high prices. I can’t begin to imagine at what price a natural diamond from the Moon (or Mars) could be sold.
The only thing I can think of that would be valuable enough to mine on Mars and return to Earth (at least with current technology) would be antimatter. Yes, you read that right, antimatter. Let’s suppose that it happens that there are deposits of a mineral on Mars that inexplicably contain antiprotons trapped as anions in the crystal lattice. The crystal lattice stably stores the antiprotons, and the richest deposits are up to one part in ten thousand antimatter by weight. Destroy the crystal, by heating it past it’s melting point for example, and the antiprotons are released for purposes peaceful or warlike. For something this potentially valuable, I think we would do whatever it took to get that first shipment (and then use it to power a spaceship to make the second shipment much cheaper). Only the fact that there’s no plausible reason this could be true spoils the scenerio.