Item from The Wheel of Time by Robert Jordan
Heartstone — the hardest substance in existence, indestructible by any known means. The Dark One's breaking of the cuendillar seals proves his growing power.
Cuendillar was created during the Age of Legends as an indestructible material. The seals on the Dark One's prison were made from it because nothing should be able to break cuendillar. The fact that they're breaking anyway is one of the most alarming signs of the Dark One's growing power. Egwene rediscovered the weave for creating cuendillar and used it to fortify the harbor chain at Tar Valon against the Seanchan attack — turning an iron chain into an indestructible barrier.
A white, stone-like material with a slight translucence. It's warm to the touch and impossibly hard — any force applied to cuendillar makes it stronger. Objects made from it are literally indestructible under normal conditions.
Also known as: cuendillar, heartstone