The Cullinan Diamond, with the weight of 3,106.75 karats, is the largest gem-quality rough diamond ever found in the history. It was discovered on 26th January 1905, at the premier no. 2 mine in Cullinan, South Africa. Thomas Cullinan was the chairman of the mine at that time and diamond is named after him. A diamond was cut into several polished gems and was presented to King Edward VII of the United Kingdom. The largest part of that diamond is named Cullinan I or the Great Star of Africa. It is the largest clear cut diamond in the world at 530.4 karats. Before the discovery of 545.67 karat Golden Jubilee Diamond, it was the largest polished diamond of any color. The Cullinan diamond was twice the size of any previously discovered diamond at approx 600 grams weight, 98 mm length, 57 mm width, and 67 mm height. The surface manager of the Premier Diamond Mining Company, Frederick Wells, immediately took it for examination after it was being handed over by a miner named Thomas Evan Powell.

Sir William Crookes asserts a weight of karats after he performed an analysis of the Cullinan Diamond. The diamond has a black spot and it has a remarkable beauty as mentioned by Crookes. The colors around the black spot changed and were very vivid due to internal strain according to Crookes. It was concluded that this stone had been broken up by natural forces and had originally been part of a much bigger diamond because one side of the diamond was perfectly smooth. Crookes pronounced the Cullinan "a fragment, probably less than half, of a distorted octahedral crystal; the other portions still await discovery by some fortunate miner." The press avidly followed the developments.

The Transvaal Colony Government purchased the diamond for £150,000 and insured it for ten times more amount. Prime minister Louis Botha suggested that as "a token of the loyalty and attachment of the people of Transvaal to his throne and person", the diamond be presented to King Edward VII. To know what should be done with the diamond, a vote was staged. The boers voted in favor of presenting the king with the diamond in the aftermath of Boer wars and the English settlers voted against such move. In the presence of a large party of guests, which includes the Queen of Norway, the Queen of Spain, the Duke of Westminster and Lord Revelstoke, the diamond was presented to the king. The king asked his colonial secretary, Lord Elgin, to announce that he accepted the precious gift "for myself and my successors" and that he would ensure "this great and unique diamond be kept and preserved among the historic jewels which form the heirlooms of the Crown".