Complex sentence: The stone was set into the crown of Louis XV for his coronation in 1722, into a new crown for the coronation of Louis XVI in 1775, and used to adorn a hat belonging to Marie Antoinette.
Split into simpler sentences: The stone was set into the crown of Louis XV for his coronation in 1722 and then into a new crown for the coronation of Louis XVI in 1775. It was also used to adorn a hat belonging to Marie Antoinette.