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In a statement, Lucara said the stone was "one of the largest rough diamonds ever unearthed".

The company owns 85% of De Beers, which produces about a third of the world’s rough diamonds and sells diamond jewelry.

Russia is the biggest producer of rough diamonds cut from hundreds of mines beneath the Siberian permafrost, where a third of the world's diamond supply comes from.

Other diamond firms in Antwerp, the world's biggest centre of trade in rough diamonds and cutting of the biggest gems, were asked to avoid Russian stones.

But even before it is in force, producers say Western customers have been shunning gemstones from Russia, the world's biggest producer of rough diamonds by volume with 30% of the market.