(IDEX Online) - Troubled Dominion Diamond Mines is suing DDMI, the majority joint venture partner at Canada's Diavik mine, alleging breach of contract.In a notice of a civil claim lodged at the Supreme Court of British Columbia yesterday, it claims DDMI, a subsidiary of the global mining giant, had failed to act in the best interests of the partnership.Dominion, the world's third largest producer of rough diamonds by value, filed for insolvency protection in April after shutting the mine (pictured) because of COVID-19.The Toronto-based company was bought in 2017 by The Washington Companies, a privately held conglomerate based in Montana, USA, for $1.2bn. It plans to sell back its stake in Diavik to a subsidiary of the conglomerate. Dominion claims DDMI, which operates the mine for a fee then shares the diamonds, has shown "willful misconduct and gross negligence" has been running the mine to its own advantage and has been demanding money in the full knowledge that it can't pay.It also claims Diavik has missed production targets and run significantly over budget.Rio Tinto responded to the allegations in an email to the Canadian broadcaster CBC News, saying: "Rio Tinto has managed the Diavik diamond mine successfully for over 15 years for the benefit of its owners, the Northwest Territories, local communities, governments, employees and suppliers."We are confident that we have acted appropriately at all times and will be vigorously defending these baseless claims by Dominion, just as we continue to protect Diavik's interests in Dominion's insolvency proceedings."