Dominion Diamond Mines Dec. 13 said the largest known gem quality diamond ever found in North America was unearthed at Diavik, a mine in Northwest Territories owned 40 percent by Dominion and 60 percent by Rio Tinto.

Weighing in at 552 carats, this yellow diamond is nearly triple the size of the next largest diamond recovered in North America, a 187.7 gem found at the same mine known as the Dia...