Why is DeBeers anti-competitive?
From 1945 to 1994, government tried several times to prosecute DeBeers for violating antitrust laws, as you can see from the slides. So from the point of government, DeBeers really is anti-competitive. Despite the fact that DeBeers retained no U.S. presence and was competitively run by South African nationals, it was still subject to the reach of U.S. law. Because in 1995 document, the justice department made its interpretation clear: “The reach of antitrust laws is not limited, however, to conduct and transactions that occur within the boundaries of the United States. Anti-competitive conduct that affects U.S. domestic or foreign commerce may violate the U.S. antitrust laws regardless of where such conduct occurs or the nationality of the parties involved.” Although DeBeers had no legal presence in the U.S. market, no U.S. directors, and even no direct sale in the United States, it sold all of diamonds in London, and let its middlemen exported them into U.S. market, which still had huge influence on U.S market on diamonds. What’s more, the most important point to prove DeBeers is anti-competitive is that DeBeers set prices for mined and rough diamonds, which means diamonds can’t have a legal and competitive market.

In 1945, President Roosevelt’s administration requested the justice department to investigate DeBeers’ existing contracts with United States.
In 1976, the department of justice filed a civil and criminal suit against DeBeers, ANCO Diamond Abrasives Corporation, and Diamond Abrasives Corporation for engaging in price fixing and customer and territorial allocation in the market for diamond grit.
In 1994, the justice department filed a suit against DeBeers and General Electric for price fixing in the industrial diamonds markets.