The Sapphire Monkey is a priceless artifact that Eliot Spencer was supposed to steal from North Korea, circa 2005. It was first referred to in a flashback during the episode The Two-Horse Job. In the episode, Eliot's high school sweetheart Aimee Martin asks him why he hadn't been as accessible as she would have liked. He explained that he was working. In the flashback, he is seen being dragged down a hallway by two North Koreans, as they demanded he tell them where the Monkey is.
The Monkey was also mentioned in a flashback during the episode The Rashomon Job. In the flashback (the events of which took place circa 2005), Eliot told his "client", Mr. Gutman, he was unable to steal the Monkey because it was in North Korea. Gutman was very disappointed, but said that Eliot could make up for it by stealing something else: The Dagger of Aqu'abi. Gutman needed this object stolen so he could move it on the black market.
The current status of the Sapphire Monkey is unknown.
According to the Commentary Track of "The Broken Wing Job", the elusive Sapphire Monkey is the object of the con the rest of the team is silently running in Japan.