The 3,106-carat Cullinan diamond which was discovered in South Africa in 1905 and eventually yielded two enormous high-quality stones was the biggest diamond ever found. It was named after Thomas Cullinan, the mine's chairman. In April 1905, it 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 then presented it to Edward VII, King of the United Kingdom.
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 are part of the Crown Jewels.
Here is a video on how diamonds are formed.