The Duchess was given the precious accessory when she became Patron of the RAF Air Cadets in 2015, which was her first official Royal military appointment, taking over from the Duke of Edinburgh.
Although Kate, 36, isn't due to return to work until September, she chose to make an appearance at the event after RAF made her an Honorary Air Commandant in 2015, passing the role from Prince Phillip to her, according to the Mirror.
With an RAF ensign draped over the high altar behind him in the abbey, Mr Welby also highlighted how members of his own family have served and been killed in the RAF - drawing attention to his own father who flew low level sorties in Burma. The Duchess of Cambridge was photographed wearing her second Alexander McQueen piece of the week, a pastel coat dress, with a hat by Sean Barrett and an RAF Dacre Brooch.
Ahead of the speech, the Queen presented a new flag, known as a "Colour" to the RAF Regiment.