The second-largest diamond ever found - a rough 2,492-carat stone - has been extracted in Botswana at a mine run by a Canadian firm.

It is the biggest find since the 3,106-carat Cullinan diamond, found in South Africa in 1905 and cut into nine separate stones, many of which are in the British Crown Jewels.

The diamond was found at Karowe mine, about 500 kilometres north of Botswana's capital, Gaborone.

Botswana's government said it was the largest diamond ever discovered in the southern African state.

The previous biggest discovery in Botswana was a 1,758-carat stone found at the same mine in 2019.

Botswana is one of the world's biggest producers of diamonds, accounting for about 20 per cent of global production.