The crown belonged to Iris' mother but when Gramorr betrayed them, he took control of Ephedia. The queen, realizing this threat, decided to send her only daughter to earth and seal the crown so that Gramorr can't use it for evil. She also used the power of the crown to scatter the Oracle Gems and sealed the crown, and in turn seal Gramorr inside the castle of Ephedia.
In the episode "Home Part II", we have seen that Iris is the only one beside the queen of Ephedia who can wield the crown and use its power even if it doesn't have all the Oracle Gems. It's also shown when Iris tried to go through the portal, the crown disappeared and returned to the throne room in crystal stating that the crown can't leave Ephedia. And Iris is now crowned princess because of the crown princess of Ephedia.
We also saw in that same episode that only Iris can wield the crown unless she transfers the power to use it to someone else using a dark spell.