It's equally valuable because it's symbolic culturally and a status symbol.
> I think the understanding that they are not rare and that the price is entirely an extortionary corporate fiction to collect huge profits will influence one's desire to pay the high price.
That's like telling people their Gucci bag actually costs $100 to manufacture and thinking it will make them stop buying them. People do things to signal social status. It's never been a rational thing so rationality isn't going to fix it.
If we flood the market with diamonds people will find the next status symbol to cling on to. You can try to stop it at gun point like the Soviets/Chinese did but that still never stopped the remaining few with status from engaging in it (see NK today).
Humans are just flawed biology who do dumb stuff and there will always be another person who will give it to them in exchange for their money and another person to desire what they now have.