Buying stones loose and having them set is a great way to save on engagement rings. In purchasing the stone separately, you can save between 10-25% when compared to a preset engagement ring. Diamond wholesalers can also help you in selecting a stone to fit your budget.
Use a slightly smaller diamond for her engagement ring. A diamond that weighs in at 0.95 cannot be visually differentiated from a full carat. That slight difference in carat weight can save on your engagement ring without her (or her friends) knowing the difference.
Stay clear of princess cuts when looking for an engagement ring. This cut requires a higher carat weight, not to mention the fact that princess cut diamonds tend to look smaller by comparison. To get the most bang for your buck, and to save on her engagement ring, look towards round and marquis cut diamonds.