The Royal Brooch, originally an earring from the Royal Jewels, is an elaborately decorated golden piece of jewelry worn by Prince Wu as a substitute for a crown, as all other coronation objects were stolen by looters during the anarchy in the Earth Kingdom.
Gun arrived in Republic City on Wu's coronation day in 174 AG with this piece of jewelry, explaining to the future king that almost all of the ceremonial objects were stolen during the riots in Ba Sing Se, including the crown. To ease Wu's panic about how people would recognize him as the king, Gun showed him the earring and urged the monarch to look at it as a royal brooch which could be worn in lieu of a crown. As the Grand Secretariat of Ba Sing Se attempted to pin the brooch on Wu's chest to convince him that it would work, Wu stormed off, though he ultimately accepted the item during the ceremony. When Kuvira denounced Wu's authority in the Earth Kingdom, establishing herself as the new ruler of the "Earth Empire", Wu pointed at the brooch to emphasize his title as the rightful ruler of the Earth Kingdom.
Later that day, Wu and Mako had retreated to the king's junior suite in the Republic City Four Elements, where the royal lamented about his brooch being a lie. To cheer him and Mako up, they visited the Little Ba Sing Se Fashion Mall, though when Wu found a boutique selling pro-Kuvira merchandise, he accused the vendor and customers of being traitors to the Earth Kingdom and told them that they should "respect the brooch".