You can argue the "blood diamond" political aspects of the diamond mining industry, but even tossing that aside DeBeers' behavior and domination and control of the diamond industry transcends any other monopoly. There's a reason DeBeers isn't a U.S. company (among many others...), DeBeers' monopolistic practices and domination and heavy handed control of the diamond market would not likely pass legal muster in the U.S.
Wouldn't a foreign company still be subject to U.S. Antitrust laws just like Microsoft was subject to similar laws in the E.U. not that long ago?
Microsoft Case
Wikipedia: Antitrust
