Normally at coronations, people wear tiaras or coronets. The then Princess Elizabeth wore a gold coronet to the coronation of her father George VI. At Elizabeth II's coronation, the audience twinkled with the effect of diamond tiaras on all the ladies.
However, there were no tiaras worn at Charles III's coronation. Instead Catherine, Princess of Wales and her daughter Princess Charlotte wore embroidered floral headpieces.
The headpiece was designed by Alexander McQueen and made by milliner Jess Collett. It featured leaves embroidered with crystals and silver bullion thread.
Here is a close-up of it:
It's an interesting innovation in coronation headwear, and we shall have to wait and see if future coronations also feature it, or if they'll go back to wearing coronets and tiaras.