I would like to take this moment to thank https://diamondssuck.com/ for preventing me from buying my wife a diamond, and getting her a much prettier moissanite instead. It's been on hn's front page several times.
Guys, buying diamonds, can complain all day about what a rip off it is (it is). But if you apprise your fiancé of the alternatives / opp costs (moissanite, man made, second hand, etc) and she still wants a new diamond, just get her a new diamond. It will make her happy for years. My wife had her heart set on a pink diamond, so that’s what I got her. I made it clear though that if I got this, she is not getting a new…
If she is apprised of the alternatives and still demands "No I want EXPENSIVE shiny thing" then it's cultural shaming factoring into the decision making. You have to reevaluate what she sees you as: an actual human being, or a provider mannequin whose validity as a partner depends on subjecting yourself to arbitrary ceremonial consumerist prostrations.