The diamond, weighing 709 carats, is now locked up in Sierra Leone's central bank in Freetown. It is one of the 20 largest diamonds ever found.

Freelance, or artisanal, miners are common in Sierra Leone's diamond-rich areas, reports the BBC's Umaru Fofana.

But there are questions over whether the community will benefit from the gemstone, he adds.

Pastor Emmanuel Momoh's discovery, which has not yet been valued, is the biggest diamond to be found in Sierra Leone since 1972, when the 969-carat Star of Sierra Leone was dug up.

It is also the 13th largest diamond ever to be found, says Mathew Nyaungwa from Rough and Polished diamond analysts.

Mr Nyaungwa adds that it is difficult to estimate a price as it is "quality not size [that] determines the value of a diamond". Our correspondent says that the discovery may have a blemish.

Last May, diamond-mining firm Lucara sold a 813-carat stone for $63m (£51m) at a closed auction in London.