Diamonds.

[butterflydreaming]

 When you see a lot of a thing all in one place, that does not mean that "it does not inherent value" or that it does not have rarity. A silo of grain does not mean that the wheat was effortless.

De Beers controls diamond prices for most diamonds sold, yes. That is why diamonds are a terrible investment choice, because unlike metals, they will not have massive price fluctuations.

Diamonds are my birthstone, but I didn't like them until I studied them. Now I can't hear an off-hand comment about them not having value without becoming wordless with rage.

Comments

[coffeedaiv.livejournal.com]

In fairness, the comment was that they do not have intrinsic value. Even then, though, as you say, since the market is largely controlled by a single entity, it does lend itself to some massive oversimplification and incorrect assumptions. One data point is not enough for understanding.
How would the price change, if there were multiple competing companies?
(or,... what if one particular clan of dwarves controlled all of the Admantine mines..?)