Marie Antoinette was Queen of France from May 10, 1774 to September 21, 1792. On April 19, 1770 it was arranged for her to marry King Louis XVI of France, which she did on May 10, 1774.
During the French Revolution of the 1790s, Marie liked to frequent a particular tower in Chateau Rochemont (thus inspiring Nancy to name her fourth case "Treasure in the Royal Tower"). For her birthday, Louis gave her a tiara with a ruby, an emerald, a sapphire and a diamond in it. However, the public saw this as her being far too extravagant while they were starving in the streets. For this reason, she refused to wear it.
Marie had the tiara dismantled and placed the ruby, the emerald and the sapphire in three medallions. She then hid the diamond in a contraption underneath her tower and hid the sapphire medallion and her journal in a secret compartment in the tower. She sent the emerald medallion to her cousin in Austria.
She was captured by Jean Le Boeuf on June 25, 1791 and everything she had was taken, including the ruby medallion and the tower, which Le Boeuf attached to his library. Louis was executed on January 21, 1793. Marie stayed in prison, was put in solitary confinement in August and was finally guillotined on October 16, 1793.
Lily Crewe's book on curses in Tomb of the Lost Queen suggests that Marie and her husband's beheading may have been the result of bad luck that came from owning the Hope Diamond.