I suppose that we tend to overlook smaller articles such as this pin, which really must have been essential when wearing full court dress.
Securing one's veil, so that it did not slip or move to look inelegant or even hide the face, must have been as important as securing one's tiara (a tiara that had slipped really would create the most unfortunate impression.....). If I am not mistaken, pins used to keep a diadem in place were often themselves jewelled, were they not?