The Cullinan is estimated to have formed in Earth's mantle transition zone at a depth of 410–660 km (255–410 miles) and reached the surface 1.18 billion years ago.
It was found 18 feet (5.5 m) below the surface at Premier Mine in Cullinan, Transvaal Colony, by Frederick Wells, surface manager at the mine, on 26 January 1905.
It was approximately 10.1 centimetres (4.0 in) long, 6.35 centimetres (2.50 in) wide, 5.9 centimetres (2.3 in) deep, and weighed 3,106 carats (621.2 grams).
Newspapers called it the "Cullinan Diamond", a reference to Sir Thomas Cullinan, who opened the mine in 1902.
It was three times the size of the Excelsior Diamond, found in 1893 at Jagersfontein Mine, weighing 972 carats (194.4 g).
Four of its eight surfaces were smooth, indicating that it once had been part of a much larger stone broken up by natural forces.
It had a blue-white hue and contained a small pocket of air, which at certain angles produced a rainbow, or Newton's rings.

Shortly after its discovery, Cullinan went on public display at the Standard Bank in Johannesburg, where it was seen by an estimated 8,000–9,000 visitors.
In April 1905, the rough gem was deposited with Premier Mining Co.'s London sales agent, S. Neumann & Co. Due to its immense value, detectives were assigned to a steamboat that was rumoured to be carrying the stone, and a parcel was ceremoniously locked in the captain's safe and guarded on the entire journey.
It was a diversionary tactic – the stone on that ship was fake, meant to attract those who would be interested in stealing it.
Cullinan was sent to the United Kingdom in a plain box via registered post.
On arriving in London, it was conveyed to Buckingham Palace for inspection by King Edward VII.
Although it drew considerable interest from potential buyers, Cullinan went unsold for two years.
The Cullinan is estimated to have formed in Earth's mantle transition zone at a depth of 410–660 km (255–410 miles) and reached the surface 1.18 billion years ago. It was found 18 feet (5.5 m) below the surface at Premier Mine in Cullinan, Transvaal Colony, by Frederick Wells, surface manager at the mine, on 26 January 1905. It was approximately 10.1 centimetres (4.0 in) long, 6.35 centimetres (2.50 in) wide, 5.9 centimetres (2.3 in) deep, and weighed 3,106 carats (621.2 grams). Newspapers called it the "Cullinan Diamond", a reference to Sir Thomas Cullinan, who opened the mine in 1902. It was three times the size of the Excelsior Diamond, found in 1893 at Jagersfontein Mine, weighing 972 carats (194.4 g). Four of its eight surfaces were smooth, indicating that it once had been part of a much larger stone broken up by natural forces. It had a blue-white hue and contained a small pocket of air, which at certain angles produced a rainbow, or Newton's rings.
Shortly after its discovery, Cullinan went on public display at the Standard Bank in Johannesburg, where it was seen by an estimated 8,000–9,000 visitors. In April 1905, the rough gem was deposited with Premier Mining Co.'s London sales agent, S. Neumann & Co. Due to its immense value, detectives were assigned to a steamboat that was rumoured to be carrying the stone, and a parcel was ceremoniously locked in the captain's safe and guarded on the entire journey. It was a diversionary tactic – the stone on that ship was fake, meant to attract those who would be interested in stealing it. Cullinan was sent to the United Kingdom in a plain box via registered post. On arriving in London, it was conveyed to Buckingham Palace for inspection by King Edward VII. Although it drew considerable interest from potential buyers, Cullinan went unsold for two years.
Cullinan Diamond