The Cullinan Diamond is the largest gem-quality rough diamond ever found,[2] weighing 3,106.75 carats (621.35 g), discovered at the Premier No. 2 mine in Cullinan, South Africa, on 26 January 1905. It was named after Thomas Cullinan, the mine's chairman.

In April 1905, the Cullinan was put on sale in London, but despite considerable interest, it was still unsold after two years. In 1907 the Transvaal Colony government bought the Cullinan and presented it to King Edward VII on his 66th birthday.

Cullinan produced stones of various cuts and sizes, the largest of which is named Cullinan I or the Great Star of Africa, and at 530.4 carats (106.08 g) it is the largest clear cut diamond in the world. Cullinan I is mounted in the head of the Sovereign's Sceptre with Cross. The second-largest is Cullinan II or the Second Star of Africa, weighing 317.4 carats (63.48 g), mounted in the Imperial State Crown. Both diamonds are part of the Crown Jewels of the United Kingdom.

Seven other major diamonds, weighing a total of 208.29 carats (41.66 g), are privately owned by Queen Elizabeth II, who inherited them from her grandmother, Queen Mary, in 1953. The Queen also owns minor brilliants and a set of unpolished fragments.