Do you know what are the largest diamonds ever found in the world? The Cullinan I, Star of Africa the largest cut diamond weighed around 520 carats with about 76 facets. It is cut from the largest rough diamond that was ever found throughout history. It weighed over 3,106 carats with a weight of 1.26 pounds The second largest diamond is the Excelsior, which originally weighed about 995 carats. The third largest rough diamond ever found was the Star of Sierra Leone which weighed about 969 carats. The fourth largest, the great Mughal weighed about 793 carats. The largest diamond found in North America is called the Uncle Sam Diamond weighing about 40.23 carats found near Murfreesboro, Arkansas which used to be displayed in the American Museum of Natural History.