The Cullinan diamond is the largest gem-quality diamond ever found, at 3106.75 carat (621.35 g, 1.37 lb) rough weight. About 10.5 cm (4.1 inches) long in its largest dimension, it was found on January 26, 1905, in the Premier No. 2 mine, near Pretoria, South Africa.
The largest polished gem from the stone is named Cullinan I or the Great Star of Africa, and at 530.4 carats (106.08 g) was the largest polished diamond in the world until the 1985 discovery of the Golden Jubilee Diamond, 545.67 carats (109.134 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 Second Star of Africa, at 317.4 carats (63.48 g), is the fourth largest polished diamond in the world. Both gems are in the Crown Jewels of the United Kingdom.
Captain Frederick Wells, superintendent of Premier Mine, one of South Africa’s most productive mines, near Pretoria, found the diamond, during his daily inspection of the mines, on 26 January 1905. During his rounds he saw a flash of light, reflected by the sun on the wall of the shaft. As he got closer, he could see a partially exposed crystal, embedded in the rock, however he initially believed it to be a shard of glass, placed by one of the miners as a practical joke. Using just his pocket knife he managed to release the diamond. At approximately 1 1⁄3 pounds, 3 7⁄8 inches long, 2 1⁄4 inches wide and 2 5⁄8 inches high the diamond was twice the size of any diamond previously discovered. Wells immediately took it for examination.