"Later! Yes, when you also have the Keys of Barad-dûr itself, I suppose; and the crowns of seven kings, and the rods of the Five Wizards, and have purchased yourself a pair of boots many sizes larger than those that you wear now..." -Saruman (to Gandalf);The Two Towers

The seven crowns probably refer to the crowns of the Seven Tribes of the Khazâd:

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