The world's biggest diamond in 13 years went on sale on Wednesday in the Belgian city of Antwerp. The successful bidder, for the gem that was mined in Lesotho, shall be named by the Antwerp Diamond Centre on Monday.

Dubbed "Lesotho Promise," the owners found it hard to give the monetary value of the piece. To get the price of a diamond rock, traders need to know how many cuts can be made out of it. The 605-carat diamond is about a third bigger than the size of a golf ball.

The rock was uncovered at Letseng Diamond Mine in the mountains of Lesotho. It is the biggest uncut diamond found since the 1993 Millennium Star from the Democratic Republic of Congo. That Millennium Star was 777 carats.

More than two thirds of the Letseng mine is owned by Gem Diamond Mining Company of Africa Ltd. The other third is owned by the government of Lesotho, a conservative African kingdom surrounded by South Africa.