Space Mining: Ore Availability and Yield

[Anthony]

Originally Posted by tshiggins

Niven also had monopole magnets, made from ferrous metals that aggregated in the absence of a planetary magnetic field.

I'm not sure how they got magnetized

I assume he was talking about magnetic monopoles, which are a theoretical particle that was reasonable to propose when he was writing those stories, but is currently believed to not exist (there are a number of primordial artifacts like that which theoretically could exist but don't seem to, which is taken as evidence that the big bang may have had a maximum temperature).