LONDON (BLOOMBERG) - 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 per cent 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 per cent markup, as buyers who have access to De Beers flipped them to traders and manufacturers that don't.

The spiking prices - and buyers' willingness to pay them - are the latest sign of roaring demand in the secretive rough-diamond trade. Just nine months ago, the industry had been brought to a standstill by the pandemic. Now, the cutters and polishers who form the backbone of the global diamond market are competing fiercely for stones.

There are several reasons behind the surge: major retailers in the United States and China are buying aggressively to keep up with strong sales, while rough diamond supply is tight because De Beers and rival Alrosa PJSC have limited supply to put onto the market. Perhaps most importantly, polished diamond prices - which have long weighed on the industry - are finally rising.

Diamonds have been a big winner from lockdowns around the globe as access to rival luxury offerings was limited. That first showed with stronger-than-expected holiday sales, from Thanksgiving through to Chinese New Year, and has since continued.

"The rough market is hot. There's enthusiastic buying across all rough categories," said Mr Anish Aggarwal, a partner at specialist diamond advisory firm Gemdax. "There are supply shortages at the moment. That's creating a sense of scarcity at every stage of the pipeline."

For some in the industry, there are worries the market could be running too hot. It takes about three months to cut, polish and sell a diamond, so stones bought cheaply at the start of the year are now being sold for a big profit. That also means those buying at today's prices are betting that polished stones will continue to appreciate in value.

And then there is the risk that the renewed consumer interest in diamonds, driven by the lack of competition from other luxuries such as experiences and travel, could cool as more economies open up.

For now, demand shows no sign of letting up. In the secondary market, where accredited De Beers and Alrosa buyers sell to other gem manufacturers, boxes of diamonds have been changing hands for premiums between 5 per cent and 10 per cent, with some select parcels even higher, people familiar with the matter said.

The fierce competition for stones has led some manufacturers to even buy polished diamonds to fill demand from retailers, rather than compete for rough goods to polish and cut for sale.

Russia is also looking to capitalise on the strength of demand and tight supplies, as it considers selling more rough diamonds from state stockpiles. The government is preparing a decree to increase planned sales of diamonds from the state repository, known as Gokhran, according to people people familiar with situation. Alrosa, is likely to bid at the sales to replenish its own inventories, which have declined to a record low.

Prices have been rising all year, with De Beers increasing its prices at almost every sale so far this year and pushing most larger stones up in price by between 5 per cent and 10 per cent this week. Some two-carat rough diamonds - the size that generally yields a 1-carat polished stone for an engagement ring - were up 10per cent.

Both of the big miners have also managed to sell down billions of dollars of stock that they built up last year, peaking when De Beers sold US$663 million (S$877.8 million) in January.

Still, last month's sale reaped just US$380 million and industry participants said they expect a similar size offering this week, because De Beers has little extra to sell. That promises to keep supply tight as retailers such as China's Chow Tai Fook Jewellery Group and Costco Wholesale Corp continue to place big orders for polished stones.

"There's genuine demand on both the rough and polished level," said Mr Aggarwal. "Ultimately, this is driven by consumers buying diamond jewelry."