A 114.63 carat diamond from the Democratic Republic of Congo failed to find a buyer after its mining company owner set up the auction to cover payroll, a union leader said Wednesday.
The outcome prompted some in the industry to wonder whether the large stone had major defects or was not fully crystallised.
"The workers are disappointed, they hoped to alleviate their poverty," said Dieudonne Kabamba, a union vice-president at the Miba diamond mining company.
Kabamba said Miba announced it put the diamond up for auction to cover late payroll and adjust other wages that had been cut by two-thirds over the last few years.
The uncut diamond, which weighs 23 grammes (0.8 ounces), went under the hammer earlier this month for $1 million (750,000 euros) but was withdrawn after failing to find a buyer for $480,000, the UN-backed Radio Okapi said.
Other diamonds sold at the auction in the central mining town of Mbuji Mayi, bringing in a total of $28,200, the station said.
A Western diamond dealer told AFP he was surprised the diamond failed to sell: "A 114 carat stone is rare and expensive."
"There's something wrong, the stone is undoubtedly ugly and very overpriced... If it's not crystallised, then it's worth nothing, whether it weighs one gramme or 100 kilogrammes."
Local union official Jean Marie Kabuya, who had not seen the diamond, said its colour or defects could have also affected the auction outcome.
According to Kabamba, the diamond was discovered in December in gravel near Mbuji Mayi.