Also called heartstone, an indestructible substance created during the Age of Legends. Any force applied to break it, including the One Power, is absorbed and makes cuendillar stronger. The seals on the Dark One's prison were made of cuendillar, though even they weakened over three thousand years. The method of making cuendillar was lost for centuries but was rediscovered shortly before the Last Battle by Egwene al'Vere.