Nightmares - S3-E7
Continuity mistake: In the very first episode Frank Reagan is getting dressed to go to Jamie's graduation ceremony. He slips on his watch and then takes a ring with a blue stone (looks sort of like a class ring) and puts it on. It is definitely and obviously a large blue stone. Then in this episode, near The End, when Frank is sitting at the bar talking to Garrett, as Frank lifts his drink in his right hand, and Garrett says "Thank you, Frank", that "class ring" has a very definite and obviously large red stone. (00:01:20 - 00:40:50)
Answer: The Seal of the City of New York.