Diamonds are worthless until someone digs them out of the ground.
Coal has obvious value. I should know. Keeping the furnace in our Upper Ohio Valley home properly stoked was my teenage job. The man who dug the coal and sold it to us knew how to price his output and my dad knew what he was willing to pay. Willing seller meets willing buyer. That’s the primary feature of a market economy. Diamonds? Apart from certain specialized industrial uses like masonry saws or some lasers, diamonds are valuable as a girl’s best friend bc of brilliant marketing, but not so much as an intrinsic good. Still, sellers find buyers even if they have to create them. But digging these things up doesn’t make them valuable, just available.