The largest diamond on Earth is the 546-carat Golden Jubilee.
(It was cut from a stone brought out of the Premier mine in South Africa in 1985. The rough stone it was cut from weighed 755 carats. It was named "Golden Jubilee" when it was presented to the King of Thailand in 1997 for his Golden Jubilee. Before that it had the uninspiring name of "the Unnamed Brown".)