One reason why diamond is deemed rare and expensive is because of how nature makes it, including the tedious process of mining it after thousands of years of nature's work on this gem.

Anything derives their value based on what the community accepts its perceived value is.

Take crypto for example. These currencies have value because the community. Throw in signature bags, clothes and shoes on this while at it.

So, one is only as expensive as what we hoomans say it to be.