If you and your future fiance are open to such things, you may want to look into an alternative like Moissanite which has most of the same properties as a diamond.
About the stone and it's history.
http://www.moissanite.com/heritage.cfm
(It's only been found naturally in space!

but the rings you'd buy would be man made. )
Here's an online jewelry store that sells them, and I've had great service with.
http://www.bellajewelryco.com/engagement.htm
When these stones first came out jewelers were unable to tell the difference. In 2001 ABC's 20/20 show did a special on them.
You can google and find the video online at some sites still. They now have tests to detect them 100%, because of the way the light refracts in them. But obviously none of our friends or family knew any different, until we told them.
My fiance felt strongly against the practices of the diamond industry, (The industry is really pretty shady if you care to do some quick research online) and actually talked me into going this route. It was hard for me to come around to the idea at first, since we've all been brought up with the "diamond is forever" spiel lately, but now I couldn't be happier!
Think of all the more important things you can do with that money. It's something you should discuss with your future-fiance first though.
Anyways that's my 2 cents.