I do make the same argument about a BMW
Total waste of money unless you have spent it on a million other things first. At least with the BMW you get incremental daily enjoyment from it. They are nicer to drive. Nobody stares at their ring all day except a newly married 50's housewife.
Some of this is just priorities, but there really are some things that make no sense outside of tradition. Yeah diamonds take a long time to make, but look how many grilled cheese sandwiches had to be made to find one that looked just like the virgin mary. Probably took thousands of years of collective grilled cheese making to arrive at that glorious moment. I still wouldn't pay $50,000 for it.
In general though I think with superbowl tickets or vacation homes or anything else its how much utility you get from something. I just doubt anyone gets that much utility out of a ring made of X versus Y if they stop to consider it.
But my wife can still have whatever she wants.