LONDON - Miners in the southern African kingdom of Lesotho have found one of the world's largest diamonds, a near-flawless white gem weighing nearly 500 carats, mining group Gem Diamonds said.

The diamond was discovered in the Letseng Mine on Sept 8, the company said on Sunday. It has been analysed by experts in Antwerp and found to weigh 478 carats, with very few inclusions and of outstanding colour and clarity and may potentially 'yield one of the largest flawless D colour round polished diamonds in history'.

Letseng is one of the most productive mines in history - four of the world's 20 largest rough diamonds have been found at the mine, including the three largest found this century.

Before it is cut into gems, it is hard to value the diamond, but a spokesman for Gem Diamonds said a similar weight stone with lesser-quality colour and clarity had recently sold for US$12 million (S$17 million).

'Preliminary examination of this remarkable diamond indicates that it will yield a record-breaking polished stone of the very best colour and clarity,' said Mr Clifford Elphick, the chief executive of Gem Diamonds.

Mr Monyane Moleleki, Minister for Natural Resources in Lesotho, an impoverished mountain kingdom in eastern South Africa, praised the productivity of the mine, one of the highest in the world at more than 3,000m.

Letseng is 70 per cent owned by Gem Diamonds and 30 per cent owned by the government of Lesotho.

The world's largest diamond is the Cullinan, discovered in South Africa in 1905. It weighed more than 3,100 carats before it was cut into more than 100 separate gems, many of which are part of the British crown jewels.