Money World’s largest blue diamond, ‘exceptionally rare’ at 15 carats, sells for $57 million April 28, 2022Scott Gleeson, USA TODAYComments Off on World’s largest blue diamond, ‘exceptionally rare’ at 15 carats, sells for $57 million The world's largest blue diamond, "The De Beers Cullinan Blue" now has a price to match its rarity. It's described as "a once-in-a-generation stone."