At the latest De Beers diamond auction this week, buyers shopping for the kind of stones used to make an average engagement ring were in for a sticker shock: some prices had ballooned by 10% since the last sale.

News of the unusually sharp increases spread swiftly through the community of 80-odd handpicked De Beers customers, according to people familiar with the discussions. But what followed was even more remarkable: within hours, some of those same stones were trading hands at a further 10% markup, as buyers who have access to De Beers flipped them to traders and manufacturers that don’t.