What is the largest stone in the imperial state crown called?

The largest stone in the Imperial State Crown is called the Cullinan II diamond, also known as the Second Star of Africa. Weighing 317.4 carats, it is a cushion-cut diamond that was part of the Cullinan diamond, the largest gem-quality diamond ever discovered. The Cullinan II is set in the band of the crown, alongside other significant gemstones.