I mean really is your day to day life any different if you buy an $8000 diamond engagement ring versus something that looks like a diamond?
There are probably a million uses for $8000 that are more sensible than spending it on a rock. There is a real practical reason to say its better to use that money to have a mind blowing experience or retire slightly earlier or pay down those student loans.
Yeah I'm old fashioned too in the sense that yes I did buy my wife a diamond engagement ring, but doing so sure as hell isn't logical. It's a remnant from a time when the mentality was that a wife was paid for with 3 cows and a goat. It implies a marriage is transactional which is the opposite of what it should be.
Diamonds are a "just because we have always done it this way" kind of expense. It's objectively silly.