LONDON (AFP) - 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, which the company said 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. "Preliminary examination of this remarkable diamond indicates that it will yield a record-breaking polished stone of the very best colour 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