Lots of things have happened in the 40 years since that article was written. De Beers do not control the market anymore the way they did before. Wikipedia says

Lots of things have happened in the 40 years since that article was written. De Beers do not control the market anymore the way they did before. Wikipedia says > De Beers's market share of rough diamonds to fall from as high as 90% in the 1980s to 29.5% in 2019 > De Beers sold off the vast majority of its diamond stockpile in the late 1990s – early 2000s and the remainder largely represents working stock (diamonds that are being sorted before sale). This was well documented in the press but remains little known to the general public. > As a part of reducing its influence, De Beers withdrew from purchasing diamonds on the open market in 1999 and ceased, at the end of 2008, purchasing Russian diamonds mined by the largest Russian diamond company Alrosa