The huge diamond was discovered by pastor Emmanuel Momoh, one of thousands who seek their fortunes in the informal mining sector that dominates the diamond-rich Kono region, the government said in a statement.
"A 706-carat diamond was presented to President Dr Ernest Bai Koroma yesterday evening," the president's office said.
"Receiving the diamond President Koroma thanked the chief and his people for not smuggling the diamond out of the country," it added, referring to the Tankoro chiefdom where Momoh uncovered the gem.
The sale of "blood diamonds" helped finance civil wars across Africa in the 1990s and often funded military dictatorships in a continent that the London Diamond Bourse estimates provides 65 percent of the world's diamonds.
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Sierra Leonean rebels allowed traders to exploit diamond mines and ship the gems abroad via Liberia.
In one of the most notorious cases, former Liberian warlord Charles Taylor was found guilty of supporting the rebels in exchange for diamonds mined by slave labour.
He died in jail in September before he could stand trial.
The death toll from Sierra Leone's civil war is estimated at 120,000 in a country that now has roughly six million people, making it one of Africa's deadliest conflicts in recent history.
The government of Sierra Leone has attempted to crack down on cross-border diamond trafficking to persuade foreign investors that "blood diamonds" are a thing of the past.
A 1,111-carat diamond was discovered at a mine in Botswana in 2015, the biggest find for more than a century.
That gem is second in size only to the Cullinan diamond which was unearthed in South Africa in 1905, at 3,106 carats uncut, according to the Cape Town Diamond Museum.
The Cullinan was cut into several gems, including two set into the sceptre and crown of the British Crown Jewels.
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