Don't go into debt for this. You need to talk to your girl and see what she is expecting. Part of the frustrating thing about diamonds is the BS they feed you about diamonds being forever and the hardest substance on earth. Not true, you can chip and scratch them. So are they really forever? I am a traditionalist in that I think the girl needs a ring, but if it were me, I'd go the colored stone route. Will and Kate have pretty much driven up the prices of sapphires for the time being, but I have several spinels that I love which are the same Mohs hardness rating as sapphire.
Also, what size is her finger? Like, my sister is a 2.75 and her .8 diamond looks like over a carat on her tiny hand. Finger size is more important than height.