Moissanite is the way to go. I got married last fall and didn't even give her the option of a diamond on principle. Nobody asks if it's real, they just think you're loaded because you're fiance is wearing a $800 rock that they think cost 20g. My friend who just got engaged had a completely custom setting designed + moissanite -- more thoughtful and less expensive than crap from your local jeweler and nobody can tell the difference. The hardest thing is getting over the massive psychological pressure that people will think you're cheap. Know what I told myself? Man the fuck up and stop being society's little bitch. Besides, two days after the wedding neither of you will ever think about it again.

edit: un-diamonds are also a great litmus test. if your girlfriend learns the real story of diamonds and still insists on having one you're probably in for much bigger problems than just the ring.