(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.