The issue is he cartel. Telling people “I overpaid for a blood diamond” and flashing them your big rock does nothing to undermine the cartel in the long run.
@Infynis@midwest.social has a great point. If you, the pleb, wears an artificial diamond and ruin the mood for the people who overpay for the blood diamonds, does devalue the status symbol.
It is the same reason why clothing brands fight so eagerly against cheap knockoffs, even if the knockoffs can be identified easily.