The Cullinan Diamond is the largest rough gem-quality diamond ever found, at 3,106.75 carats (621.35 g or 1.3698 pounds), found in Cullinan mine in the Transvaal in 1905. It was bought by the Transvaal government for 150,000 pounds and presented to King Edward VII as the largest diamond known. It has subsequently been cut into 9 large stones.
The largest polished gem from the stone is named Cullinan I or the Great Star of Africa, and at 530.2 carats (106.04 g) was the largest polished diamond in the world until the 1985 discovery of the Golden Jubilee Diamond, 545.67 carats (109.13 g), also from the Premier Mine. Cullinan I is now mounted in the head of the Sceptre with the Cross. The second largest gem from the Cullinan stone, Cullinan II or the Lesser Star of Africa, at 317.4 carats (63.48 g), is the fourth largest polished diamond in the world and is also part of the British crown jewels, as it forms a part of the Imperial State Crown. Both gems are on display at the Tower of London, as parts of the Crown Jewels of the United Kingdom. In 1905, transport from South Africa to England posed a bit of a problem with regard to security. Detectives from London were placed upon a steamer ship that was rumoured to carry the stone, but this was a diversionary tactic. The stone on that ship was a fake, meant to attract those who would be interested in stealing it. The actual diamond was sent to England in a plain box via parcel post.
Rumours abound of a "second half" of the Cullinan diamond, as there are certain indications that the diamond was part of a larger crystal. It is suggested that before Frederick Wells sold the diamond to Sir Thomas Cullinan he broke off a piece which sized in at about 1,500 to 2,000 carats (300 to 400 g). If this were true, the original Cullinan diamond would have weighed approximately 5,000 carats (1 kg).
It was found by Frederick Wells, surface manager of the Premier Diamond Mining Company in Cullinan, Gauteng, South Africa, on January 25, 1905. The stone was named after Sir Thomas Cullinan, the owner of the diamond mine. Sir William Crookes performed an analysis of the Cullinan diamond and mentioned among others its remarkable clarity but also a black spot in the middle. The colours around the black spot were very vivid and changed as the analyzer was turned. According to Crookes this pointed to severe internal strain. Such strains are not uncommon for diamonds, and have actually resulted in causing diamonds to explode when reaching the surface, or even in the pockets of the miners due to the exposure to the miner's body warmth. The stone was bought by the Transvaal government and presented to King Edward VII. It was cut into three large parts by Asscher Brothers of Amsterdam, and eventually into some 11 large gem-quality stones and a number of smaller fragments. At the time, technology had not yet evolved to guarantee quality of the modern standard, and cutting the diamond was considered difficult and risky. In order to enable Asscher to cut the diamond in one blow an incision was made, half an inch deep. Then a specifically designed knife was placed in the incision and the diamond was split in one heavy blow. The diamond split through a defective spot which was shared in both halves of the diamond.