Interesting theory, however, as far as I know, you can't sue for damages for a broken engagement in Netherland, yet diamond engagement rings have never taken off here.

Besides, if you need that kind of collateral before you can trust a guy, maybe you should be looking for another guy in the first place. It also doesn't explain why people who don't have premarital sex still want diamond rings.

So all in all, not a great theory.