* Mine owned by Petra Diamonds may take 5-10 years to clear debt
* Diamond miners face price pressure, synthetic competition
The owner of one of the world's most famous diamond mines could be about a decade away from clearing its multi-million-dollar debts, in a sign of the struggles facing an industry assailed by synthetic rivals and uncertain demand.
Petra Diamonds bought Cullinan in 2008, aiming to breathe new life into the South African mine renowned for yielding the largest rough gem diamond ever found – 3,106 carats – and being the world's main source of rare blue diamonds.
The London-listed miner, which acquired Cullinan from industry leader De Beers, borrowed heavily to revamp the facility and began mining a new section of ore last July.
Petra told Reuters its debts from the mine stood at around 65 percent of its overall $650 million in borrowing, which would represent about $420 million.
Cullinan's general manager Juan Kemp added that it could take "between five and 10 years" from the opening of the new section to clear the debts related to the mine. That goes beyond the 2022 maturity of Petra's bond notes.
The company says it expects to generate free cash flow this year – a target it had hoped to reach in 2017 before being derailed by strikes and construction delays – and start reducing its debts.
Kemp said one thought kept him awake at night: "When will we get that next big stone?"
Ben Davis, mining analyst at Liberum, said the diamond prices Petra had achieved were below market expectations.
"Everyone is very much hoping, for the sake of the equity holders and debt holders, it will deliver more higher-quality stones," he added.