That had a lot to do with De Beers blacklisting any buyers of used diamonds, so used stones would end up their only supply. Meanwhile running "Diamonds are Forever" campaigns that would paint any such recycling as the actions of sleazy dishonest pawn brokers. You wouldn't sell Grandma's precious heirloom to one of those people now, would you? Nowadays I'm not sure the cartels have the power they used to, but whatever's left does still seem pretty disproportionate.