The world's largest gem diamond ever found is the 3,106 carat Cullinan Diamond. It was discovered on 26 January 1905, during an inspection of the Premier mine in Pretoria, South Africa, by superintendent Frederick Wells.
The diamond was named Cullinan after the mine's owner, Sir Thomas Cullinan. It was cut by the Asscher Diamond Company of Amsterdam. Joseph Asscher, head of the company, is said to have studied the huge diamond for over six months before deciding how to best divide it.
Eventually, the Cullinan was cut into 9 major gems, 96 smaller brilliants, and 9.5 carats of unpolished pieces. The largest of the cut stones is called the "Cullinan I", also known as the "Star of Africa I,". At 530 carats, it is the world's largest-cut fine-quality colourless diamond.
Additional information: According to the Guinness Book of World Records, the largest faceted (cut) diamond is an unnamed Fancy Black, containing small red diamond crystals. It weighs 555.55 carats and was polished into 55 facets over several years and completed in June 2004.