A decade ago supermodel Naomi Campbell was embroiled in controversy over a gift of alleged “blood diamonds” from former dictator Charles Taylor. The African conflicts that such stones helped to finance are now mostly over. These days, the threat to natural diamond sales comes from synthetic stones.

Danish jewellery retailer Pandora has promised its wares will only contain diamonds created in a laboratory. The natural diamond business was already struggling.

The industry has established certification of “clean” gems supposedly free from the taint of financing warfare. Known as the Kimberley Process and backed by 56 countries, the provenance system covers most freshly mined diamonds. Some human rights campaigners, such as Global Witness, have complained the certification has sometimes been applied laxly.