
One guy has suggested synthetic diamonds...cheaper than mined ones and colored (much rarer in nature) are easier to come by for less:
http://www.apollodiamond.com/
http://www.chatham.com/
http://d.neadiamonds.com/
....and then there's Moissanite. (no, it's not Coligny's self discovered mineral) Space diamonds...ooooh! This stuff sounds quite nice as well if you have one of them rare womens that is rational.(or doesn't know any better)
http://en.wikipedia.org/wiki/Moissanite