Franz Houtman's stone

In the series Fena: Pirate Princess, Yukimaru, and the people of his island were given a stone from Franz Houtman, the father of Fena Houtman. They were only given the instruction to protect it with their lives, and are unaware of what importance the stone holds. Fena is also unaware, but they assume she may know the way to "unlock it's secret."

In the episode "The Mystery of the Stone," the bürgermeister reveals that it was created in 1436 in France, and was the property of Joan of Arc, despite her being dead for five years by that point.