My advice? Unless she has already let on that a diamond is an absolute must, don't do it. Think of all the other wonderful ways you can spend $X thousand dollars. If I had known what I know now, I'd have spent the money on a vacation to create some memories instead of sinking a few thousand into a rock that -- while beautiful -- isn't as rare as the industry would have you believe and sometimes is harvested under questionable circumstances.
We felt much the same way -- especially about the way that DeBeers et al behave. For my ring, I wanted something simple, with no stone -- rings that aren't smooth just get caught on things for me. For her the ring is more symbolic than anything else, not a contest to see who can sport the biggest rock. We ended up getting a vintage ring at a consignment / antique mall shop. The diamond in it isn't particularly flashy, but she got to pick out what she liked. Sure, some people say the woman can always take a ring back and get what she wants, but it's kinda Not The Same. I suggest shopping for the ring together. The bit one sees in old movies where a guy buys a ring and the woman doesn't have a say is an artifact of gender roles that are substantially different these days.
