Diamond exports from the Democratic Republic of Congo's main company, Miba (Miniere de Bakwanga), fell by 80% in the past year, official figures show.
Some 10,000 miners work illegally at Miba's main mine
A BBC correspondent says the main cause is a violent conflict involving illegal miners at the state-owned firm's largest mine in the Kasaai region.
Equipment worth $10m is not being used because of insecurity at the mine.
As a result of the slump in output, Miba's 6,500 employees have not been paid for more than four months.
From June to December 2006, Miba exported 545,000 carats of diamonds, compared to more than 2.5m in the same period in 2005, according to official figures leaked to the BBC.
Miba head Gustave Luabeya says overall output has fallen by some 50%.
He blames the collapse on DR Congo's landmark elections, held in July and October 2006.
The dragline is no longer in use
"It was electoral period from June. Since Kasaai is an opposition stronghold, it was not easy for us," he said.
He also points to the murder in June of South African engineer Mike Baby, who was operating a new $10m dragline - a sort of crane that scoops the diamond-rich soil out of the mine at high speed.
Miba purchased the dragline, hoping it would double or even triple production.
Mr Luabeya says the company that sold the dragline to Miba refuses to send another expert until security at the mine is improved, so the dragline is not in use.
The BBC's Arnaud Zajtman in DR Congo says it is not clear who killed the engineer but there have been several other violent incidents since then.
Capital of misery
Only four days ago, a Miba guard was shot and wounded at night.
About 10,000 clandestine miners illegally enter Miba's open mine every day.
And many saw Miba's purchase of the dragline as unfair competition, our correspondent says.
They also feared that one day, they would lose control and not be tolerated on the mine any more.
Union Leader Alexandre Ngandu Ntumba told the BBC that since they are not paid, the employees are not able to pay for school fees for their children and that they might launch a strike if no solution is found soon.
Mr Luabeya said he was trying to source the money to pay the workers.
Human rights group Cojeski says that the entire economy of the Kasaai capital, Mbuji Mayi, is being affected by the problems at Miba, the only large company in the city of three million people.
Our correspondent says that more than ever, the world's diamond capital is also one of the world's capitals of misery.