Gandalf: Does it not worry you that the last of the Dwarf-rings should simply vanish along with its bearer? Of the Seven Dwarf-rings, four were consumed by dragons, two were taken by Sauron before he fell in Mordor... the fate of the last Dwarf-ring remains unknown, the ring that was borne by Thrain.
Saruman: Without the Ruling Ring of Power, the Seven are no value to the Enemy. To control the other Rings, he needs the One. And that Ring was lost long, long ago. It was swept out to sea by the waters of the Anduin.