Rio Tinto announced it has unearthed one of Canada's largest-ever, gem-quality rough diamonds at its Diavik diamond mine in the remote Northwest Territories, a 187.7-carat stone called the Diavik Foxfire.

The gem, now being showcased at Kensington Palace in London, will later be assessed at Rio Tinto's diamond sales and marketing hub in Antwerp. Rio Tinto, which expects the stone to produce at least a 50 carat polished diamond, did not estimate the gem's value.

Discovered in August, the diamond is the second or third largest rough diamonds mined in Canada and the largest ever for a Rio Tinto diamond mine, a spokeswoman said.

Rio Tinto operates the mine and owns a 60 percent share, with Dominion Diamond Corp holding the remainder. Late last year, Rio approved the $350 million expansion at Diavik, with production seen starting in late 2018.

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The mine produces six to seven million carats annually of mainly large, white gem quality diamonds. Production began in 2003 and is expected to continue until 2023.

Two weeks ago, a small Canadian diamond miner found the world's second-biggest gem quality diamond from its mine in Botswana. The 1,111-carat stone, slightly smaller than a tennis ball, could sell for more than $60 million, the chief executive of Lucara Diamond Corp has said.

One day later, the company announced the recovery of an 813-carat and 374-carat stone from its Karowe mine in Botswana.

The world's biggest ever gem-quality diamond is the Cullinan, a 3,106-carat stone found in 1905 at the Premier mine in South Africa. It was cut into several polished gems, the two largest of which are part of Britain's crown jewels.