A model holds a white diamond weighing nearly 500 carats in this undated handout photograph released in London on September 21, 2008.(China Daily/Agencies)

JOHANNESBURG, Sept. 21 (Xinhua) -- Gem Diamonds, a London-listed mining firm, said on Sunday it had recovered a 478-carat diamond from its mine in Lesotho: the 20th largest rough diamond ever found.

The discovery of the gem had the potential to become one of the largest round-cut diamonds in the world, was made on September 8 at the Letseng mine in Lesotho, the South African Press Association quoted the company source as saying on Sunday.

"Preliminary examination of this remarkable diamond indicates that it will yield a record-breaking polished stone of the very best color and clarity," the company's Chief Executive Clifford Elphick said in a statement.

The diamond, which has not yet been named, has the potential to yield a 150-carat polished stone, a company spokesman said.

That would be far bigger than the 105-carat round-cut Koh-i-Noor diamond seized by Britain from India in the 19th century and now part of the Crown Jewels.

It would still only be a fraction of the size, however, of the Cullinan diamond discovered in 1905, which was 3,106 carats when recovered and yielding a teardrop shaped diamond of 530 carats: the Great Star of Africa.

The Letseng mine is owned by a mining company that is 70 percent owned by Gem Diamonds, with the remaining 30 percent held by the Lesotho government.