From the Glossary of The Wheel of Time

An indestructible substance created during the Age of Legends. Any known force, including the One Power, used in an attempt to break it is absorbed, making the heartstone stronger.
(This applies to The Wheel of Time universe)

In the time in which the novels are, heartstone objects are extremely rare (and valuable). Seven of these objects are focal points for the seven seals of the Dark One's prison. They are discs with the Aes Sedai symbol from the Age of Legends. They are also breaking, one at a time (although this is supposed to be impossible - just another advantage of being the root of all evil) as the Dark One comes closer to being released again. Some people can feel the Dark One's evil through these objects, as they are the only link between the real world and the seals.

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