Of course it's not - but right now the value does not follow the real supply and demand curve, but is fixed by DeBeers, et al. We have lots and lots of diamonds, and if not for a clever decades long marketing campaign, they would be mostly worthless. Going to space to get more diamonds (which we don't need, because we already have too many of them) would be a colossal waste of resources.

This exactly. DeBeers changed the way Americans value diamonds, adjusting the demand curve upwards, so Americans expect to pay more money for diamonds.

Value isn't an intrinsic quality of anything. Humans value things, usually based on their relative utility.

(Likewise, quality isn't an intrinsic value of anything, either. Humans assign qualities to things. There is a buzzword called "quality" which means "homogenized reproducibility")