LONDON, United Kingdom — The latest sales data from De Beers reinforces why this is one of the worst years for the diamond industry in a long time.

The Anglo American Plc subsidiary reported sales on Thursday that showed demand for rough diamonds is continuing to plunge as polishers and traders refuse to buy stones when they can’t make a profit.

The mining company holds ten sales events each year in Botswana, where its chosen buyers — known in the industry as sightholders — are given a box containing plastic bags filled with diamonds. In the past three sales, De Beers made less than $300 million, which is unprecedented in data going back to 2016.

The crisis in the diamond industry stems from an oversupply of polished gems, which has depressed demand for rough stones. Much of the polishing and trading industry is based in India, where companies have been squeezed by tight bank financing and currency fluctuations.

However, it’s unlikely that shoppers will see much change in jewellery prices at the retail level. Those prices tend not to fluctuate and reflect other costs, like marketing and labor.

De Beers sold $295 million of diamonds in its eighth sale of the year, 39 percent less than a year earlier, the company said on Thursday.

The mining company has tried to counter the weak market by giving buyers more room to manoeuvre. In normal times, sightholders have to accept the price and quantities of stones they’re offered.

But with many sightholders now struggling, De Beers has allowed them to refuse half the stones in many of the diamond parcels, according to people familiar with the situation. They can also sell back some stones to De Beers on favourable terms.

“As we approach what is traditionally a quieter time of year for the diamond industry during the Diwali holiday, we have again offered our customers flexibility during this sales cycle,” De Beers Chief Executive Bruce Cleaver said Thursday.

RBC Capital Markets expects profit will fall by about 40 percent this year at De Beers. The company’s decision to let buyers reject stones will help reduce the oversupply in the market, which should eventually allow prices to recover, RBC said.

“If history is any guide, De Beers removing meaningful volumes from the market, as they did in 2008 and 2015, usually allows for the market to tighten substantially once conditions normalise,” said Tyler Broda, an analyst at RBC.

By Thomas Biesheuvel; editors: Lynn Thomasson, Dylan Griffiths and Gordon Bell.