Sigh.
While I agree with your investment analysis of the value of diamonds, you are missing the whole point of why you buy the ring in the first place Matt. It's not about future values, or long term investments. It's not something you will want to take a look at years later and get upset because it lost value, because it loses value the moment you buy it.
A wedding ring is a symbol of your love for a woman, it's tradition, and the fact is that most women today are groomed to expect the diamond engagement ring and they want that ring. Sure a car might be a better monetary investment, but you have to give the woman what she wants, not what you think is best for her. Fact is most women enjoy showing off their diamond engagement ring, it's bragging rights. My ex hardly ever wore jewelry, but when I bought her an expensive beautiful engagement ring she wore it every chance she got, she showed it proudly to everyone she could. She didn't do that because she liked wearing rings, she didn't do it because it was worth alot, she did it because it was a beautiful symbol of how I felt about her. A car wouldn't have gotten the same reaction.
If I ever get married again and if she wants a diamond ring you can be damn sure she's getting a ring she can be proud of. Because if I'm marrying her then she's worth it.