Buying a diamond for someone can be meaningful even if diamonds aren't actually scarce.

Diamonds are significantly less scarce in the natural world than their price implies, due to cartels.

But buying a natural diamond is expensive, because the expected supply is scarce.

So even if they aren't actually scarce, they can still be a calibrated signal of your investment to the receiver.

Doing something conspicuously expensive is one hard-to-fake way for someone to show how much they value you.