Worry less about the rock and more about the overall ring. To use a metaphor that might resonate with members of the forum - you wouldn't buy top of the line fabric and have it made into a suit by just anyone. I think the same should be true of jewelry, including an engagement ring. Taking a very expensive rock and putting into a stock, run-of-the-mill setting is silly. Instead, find a custom jeweler, get some advice, talk about what sort of style your would-be fiance has, etc. If you let a quality craftsman make you a ring from scratch, you'll end up with something significantly more attractive, without having to worry so much about getting a big, perfect, diamond.