The Cullinan Diamond, weighing 3,106 carats, is the world's largest (one carat equals about 200 milligrams). It was discovered on January 25, 1905, at the Premier Diamond Mine, in Transvaal, South Africa. Named for Sir Thomas M. Cullinan, chairman of the Premier Diamond Company, it was cut into 9 major stones and 96 smaller stones called "brilliants." The total weight of the cut stones was 1,063 carats—only 35 percent of the original weight.
The largest of the major stones, which remains the largest cut diamond in the world, is called Cullinan I. Also known as the "Greater Star of Africa" or the "First Star of Africa," it is a pear-shaped diamond weighing 530.2 carats. It is 2.12 inches (5.4 centimeters) long, 1.75 inches (4.4 centimeters) wide, and 1 inch (2.5 centimeters) thick at its deepest point. It was presented to Britain's King Edward VII in 1907 and was set in the British monarch's scepter.
Cullinan II, also known as the "Second Star of Africa," is an oblong diamond which weighs 317.4 carats. It was set in the British Imperial State Crown.
Sources: Argenzio, Victor. Diamonds Eternal, pp. 40-43; Field, Leslie. The Queen's Jewels, pp. 72-75; How In the World?: A Fascinating Journey Through the World of Human Ingenuity, pp. 144-45.