Pelosi, who was dressed in all black presumably to convey the somber nature of presidential impeachment, wore a large brooch representing the Mace of the Republic, which symbolizes the legislative authority of the House of Representatives.
Every day the House is in session, the sergeant-at-arms carries the ceremonial mace into the chamber and places it to the right of the speaker. According to the Office of the Historian, it’s made of three parts: a bundled shaft of 13 rods, a silver globe and an eagle with spread wings. The bundled rods resemble fasces, used by the ancient Romans to symbolize that people are stronger together than they are separately.
Here’s an image of the sergeant-at-arms carrying the mace into the House of Representatives in 1941.
And here’s Pelosi again on the House floor during Wednesday’s proceedings, wearing her brooch.
House rules state that the speaker may direct the sergeant-at-arms to pick up the mace and use it to restore decorum when things get unruly, by “presenting it” to the offender. In 1858, for example, the sergeant-at-arms held the mace to restore order when Wisconsin Republicans John “Bowie Knife” Potter and Cadwallader Washburn ripped the hairpiece from the head of William Barksdale, a Mississippi Democrat.
The current mace has been in use since 1842 (so it was there for the hairpiece incident).