Not everyone wants to spend $20,000 on a rock so that their spouse isn’t “embarrassed in front of her friends.”
I am a person who doesn't wish to spend $20,000 on a rock so that my wife won't be embarrassed in front of her friends. And that is why I chose to marry a woman who thinks that spending $20,000 on a rock is a silly thing to do.
That was my point. If you married someone who doesn't like diamonds, fine.
The original comment was about "shenanigans like buying something else and passing it off as a diamond". Isn't there anything your partner likes? Would you buy something else because you don't wish to spend $X on Y, "passing it off as Y", and embarrass her in front of her friends?
That's not the argument, AIUI. The argument is "Go ahead and spend $FOO on a rock, just be sure not to lie about how much you spent".
Same thing, just that's more convincing to some people/shows it's not 'cheaping out', it's just a better deal.