Kate Middleton opted for a vibrant teal outfit instead of traditional green for St. Patrick's Day.
The brooch is officially known as the Irish Guards Brooch and features a single emerald set in the center of its leaves.
Some say Queen Alexandra was the first owner of the brooch, while others say it was given to Princess Mary by the Irish Guards regiment in 1961.
In 2012, the late Queen Elizabeth received a shamrock brooch with a diamond bouquet on an emerald ribbon.