rough diamond

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.

Most proposals under consideration by the G7 now focus on rough diamonds of 1 carat and above initially.

"There are literally no buyers" of rough diamonds now, he said.