Okay so. There’s a lot of factors that go into this; make sure it’s a gem she likes, not just diamond for the sake of diamond. Get synthetic; the entire “natural” diamond industry is a scam. It doesn’t hurt to go with something custom, but keep in mind one important thing that I should have.
If your fiance is somebody who tends to lose things, go cheaper and get copies. I learned this the hard way, and it’s a very painful lesson to learn. If she’s some kind of scatterbrain or klutz, not to be rude but you need to be clinical about this, you’re better off spending a couple of hundred bucks and getting like five of them and just putting the extras in a safe or something.
Because honestly the worst thing in the world is having her in tears because she lost something that important, plus the disappointment you will inevitably feel when she lost something so expensive. I know this is probably a niche issue, but you have to be objective about these kinds of things.