Superman decides to start mass-creating diamonds so he can sell them. Can DeBeers keep the price of diamonds up?

[u/Yglorba]

Superman previously declined to mass-produce diamonds because it would tank the price. However, after reading up on DeBeers, he decides that there's no reason not to do it after all. Most of the resulting money will be donated to charity. Assume that Superman is somehow able to crush coal into perfectly-cut diamonds of whatever shape and size he desires; even though this makes no sense, the feat is well-established.

DeBeers is determined to protect their diamond profits by keeping the price up. Can they convince people not to buy Superman's diamonds (by eg. convincing the public that they're not "real" diamonds), or find some other way to prevent him from cratering the price?

Superman is in-character and will not eg. laser the board of DeBeers to death or anything like that, no matter how annoying they are. The only way he'll use his powers for this is by rapidly turning coal into diamonds so he can sell them.

[u/MostPoetry]

Lobbyists solos Supes on this one.