Cuendillar, or heartstone, is a substance made by the One Power. Nothing made of cuendillar can be broken by the One Power, not even by Balefire. Its making was known in the Age of Legends, but it was lost after the Breaking of The World, and in the Third Age its manufacture is unknown.
The seals to the Dark One's prison were made of cuendillar, but somehow they have lost their indestructability over the ages. All other pieces of cuendillar have remained intact since their creation.