For most of its history De Beers could not operate legally within the United States, because it flagrantly violated US antitrust laws. Worse, it refused to cooperate with the United States in both World Wars, leaving the US desperately short of industrial diamonds. Then it bought up all the diamonds that the Soviet Union could produce once it found diamond reserves, giving that horror state a nice source of hard currency for its mischief. It engaged in unethical anticompetitive practices, forcing possible competitors out of business through predatory pricing.
Finally, Canada and Australia had enough diamond production to ignore De Beers. It gave up its monopoly and now controls only about a third of world production. Interestingly, prices for diamonds are now higher than they were when De Beers held its monopoly. But the prices are now very variable. Very expensive diamonds are a highly speculative investment and inexpensive diamonds aren’t an investment at all.