The Wheel of Time - The Call of the Horn
Cuendillar (pronounced CWAIN-de-yar), also known as heartstone, is an almost indestructible substance created during the Age of Legends. It absorbs any force attempting to break it, including the One Power, and is believed to become stronger as a result.
Because the making of it has been lost since the Breaking of the World, cuendillar is extremely rare and valuable. The Seanchan High Lord Turak had a vast collection of it, which both contributed to and signified his power. The original Domination Band was made of cuendillar. Cuendillar was the material that was used to make the seals to the Dark One’s prison. Not even balefire can damage it. Some of the Great Seals, however, have weakened, and some of them have been found in pieces – something unprecedented in history. It is unknown what power caused them to break, but it is generally believed to be related to the Dark One breaking loose. It is believed that the True Power is one of the only things that can destroy cuendillar.
Egwene al’Vere was able to reconstruct the weave for making cuendillar from iron by using flows of Earth, Air and Fire. Not only has this allowed the Rebel Aes Sedai a source of income for their siege on the White Tower, but it has factored into their plans heavily. They have also used this new weave to make the harbor chains of Tar Valon into cuendillar so that only small vessels may enter the harbors. Leane Sharif is considered second best at producing cuendillar, Egwene being the best. Bodewhin Cauthon also has great skill at making cuendillar.