the black stones

['thul]

Some few stones were made to permit traveling, but most were made to store memories. As for how they work? isn't it obvious? Magic.

The reason only the most powerful used the stones to travel is that they were dangerous to one and all, even Elderlings. As for magical strength, that is quite probable, as the younger elderlings

end of BoD class spoiler

have until recently lacked access to Silver.