Diamond
For many decades the trading of diamonds was controlled by the De Beers group of companies, who controlled most of Africa's rich diamond mines. However, in the late 1980s and early 1990s, new diamond mines opened in Canada and Australia that De Beers wasn't able to control.  When the USSR collapsed in 1991, many cheap Russian diamonds entered the market, overwhelming De Beers and their efforts to control it.  De Beers still runs most of Africa's diamond mines, but their mines now only produce about one third of the world's diamonds.