I've always wondered what was the true meaning of the drastic shift in Hank's interests when he gets obsessed with minerals in Breaking Bad. There are answers floating around the web saying "minerals are like blue meth" showing how deeply Hank was getting involved with the case. True, however, I noticed that many times Hank was shown as a less educated guy feeling "dumb" whenever Walter talked science. Is it possible that Hank was actually admiring Heisenberg's intelligence in a way just like Gale did and he was trying to become "smarter" like Heisenberg when he was failing at catching him?
I don't think that Hank's interest for rocks and minerals has a particularly deep meaning (but that's just my opinion).
If I remember correctly, in one of the interviews of the Breaking Bad's cast contained in the BluRay version of the TV Series (might have been Hank's or Vince Gilligan's), it was stated that the choice of rocks was considered appropriate because it best represented both Hank's moral and physical condition.
Since the "accident" that has forced him to stay in bed, almost paralyzed from the waist down, Hank has not been able to fight back and, as Tom wrote above, chose to concentrate on a "dull" hobby to withdraw from real life. He's (almost) literally and metaphorically as inert and still as a rock.
Edit: it was not Gilligan's nor Norris's interview. It was a comment by Michelle MacLaren (Executive Producer/Director of the show) that said (talking about Hank's) "Many metaphors you can discuss. He's hit rock bottom. They're inert, as is his body" [Season 4 ~ Special Features "Thirty-Eight Snub" - circa 1:50]. Can't really remember if there actually were other comments by others... just too many hours of special contents to watch ;)