The brooch has a single emerald set in the center of its gold leaves.
In 2011, Princess Diana wore the brooch to her first royal event, and since then, Kate has worn it every St. Patrick's Day.
Queen Alexandra, who started the royal habit of giving the Irish Guard real shamrocks, may have held the brooch.
Others believe the brooch was crafted by Cartier and given to Princess Mary, the Princess Royal, in 1961.
Kate is the only royal to wear the brooch regularly.