Is she the kind of girl who absolutely insists on a diamond? Diamonds aren't even the traditional stone of choice for a betrothal. It was a hugely successful marketing ploy in the 50s by De Beers which made us think that way. So if she happens to have a favorite gemstone that isn't a diamond, you could probably even go with that. Not to mention since carats are based on weight, and diamonds are denser than other gems, you could get a bigger stone for your carat.