> They aren't that rare but the concentration is so low and they are always mixed in with other elements so it takes a lot of processing. I think the original

> They aren't that rare but the concentration is so low and they are always mixed in with other elements so it takes a lot of processing. I think the original meaning of "rare" may have been in opposition to dense, rather than meaning scarce. They are not densely concentrated in the substrate they're taken from as you say. From M-W rare, "3. marked by wide separation of component particles."