All good points. I think the real takeaway here though is if the price of a diamond really floated to its fair value, we'd be buying rings with something else on them. Which satisfies most of your list, but in addition, it would be more rational because what you buy could actually be justified as some sort of investment and not merely burning your money in essentially a grand century long con job.

Imagine instead buying a plastic ring for your beloved which would have engraved the account number and sum total of your engagement 401k you bought for the love of your life.