I've geared this post more towards the fellas, but this can apply to ladies as well since we often plant these sort of ideas in our guy's head. When it comes time to purchase an engagement ring, consider browsing estate sales and antique jewelry stores instead of buying a brand new ring.
Go green and check out this site of fabulous antique rings:
http://www.antiquejewelryforsale.com/There are countless sites similar to this one, but this one looked really neat.
I love the idea of having a ring that is 80-100 years old. Now that is some history! I also love it when a guy uses a ring that is already in the family. You can have it re-set, add stones to it, or present it as is. I can't think of anything more special.
If you do decide to buy a new ring, please make sure that it is a conflict-free diamond. The concept of a "blood diamond" is unfortunately a very real thing, not some scheme Hollywood devised to sell movie tickets. Having traveled to Africa twice, I can assure you it is a reality for many people who are forced to work in this industry under horrific conditions.