they're not rare, De Beers manually controls the market price by limiting the amount of diamonds on the market.

[u/valsagan]

[u/Raymat11]

Just because diamonds aren't naturally rare on Earth, doesn't mean they can't be naturally rare in whatever D&D setting you're playing

[u/grimmlingur → u/Raymat11]

Also diamonds have not been plentiful for most of human history. Diamonds have been naturally expensive because they were hard to find for a long time before humanity became good at finding them and their value had to be artificially inflated to keep them expensive.