De Beers holds a quasi-monopoly on the diamond business. And if any other firm starts its own, De Beers offers to buy it.
They control so much of the diamond market that, should the new business refuse to sell, they could flood the world with diamonds with very low price, and lead the competitors to bankruptcy.
Since they are pratically the only producer, they set the highest profitable price(not the highest price, but the price that brings in the highest profit).
In more competitive markets, firms have a lower markup, that is, they charge prices lower than that which would bring them the maximum profit.
For keeping prices that high, they offer a quantity of diamonds much below their diamond capacity.
Still, we have to keep in mind that we are talking about diamonds. These are really unnecessary goods for consumers (and those who buy diamonds for medical, industrial use pay lower prices, because the rocks needn't be flawless or anything like it). So what if they are very expensive?
The only reason why they are expensive is because people are willing to pay for them. If people wake up tomorrow and decide diamond jewelry is as valuable as onyx, prices will drop, because no-one will accept De Beers's absurdly high prices.
So no, I wouldn't say that, based on this, De Beers is "evil".
However, it is also said they have very unlawful ways of defending their properties (little private armies who which threaten even passerbies), of treating their employees in inhumane ways and not putting much value on people's lives. If those charges are true, then it is true that the company is very immoral.
As for holding a monopoly on diamonds and charging high prices (which are only high because people want them to be), there doesn't seem to be anything wrong.
