Oh, jennings rampage, you are a fool. First of all, you can get non-South African diamonds. Try Australian or better yet, man-made. Both make lovely non-massacre-inducing trinkets. Second, you are not buying her sex and/or slavery. The ring (or rather a dowry/gift of financial substance) is meant to secure your commitment. How do we as women know that you won't flake out on us. We know it happens.
Don't think of it as retainer. She needs collateral.