Foxfire

Depending on who you believe, the largest, or second or third largest, diamond found in North America. Found at the Diavik Mine in the NWT - I've searched, I can find no larger finds in North America.

187.7 Carats.

Link: http://business.financialpost.com/news/mining/rio-tinto-finds-187-7-carat-diavik-foxfire-one-of-canadas-largest-diamonds-at-arctic-mine