I've been searching for a decent informative webpage on gemstones for ages. I've had this plotbunny for a novel with a magic system based on gemstones gnawing away at me for a long, long time. I never had enough info to write it decently, though. The World of Gemstones (link from Diana-san) is a decent resource, and I might just start writing that story. After I've finished the one I'm writing now, of course. (Heh, me finish a story before starting another... that's a first.)