The Low King cannot be Low King without this artifact.
Tragedy came however when the scone was apparently stolen from its keeping place.
The belief is that the truth was once something solid, like a mineral, and the last piece of it was hidden by Agi Hammerthief and baked into the Scone. Because the Scone contains a grain of truth it is said to burn red-hot if a lie is told in it's presence, a fact than came in useful when Low King Rhys wished to interrogate Dee.
The stone is also referred to as "the thing and the whole of the thing."
A copy of the stone is kept at the Museum of dwarf bread in Ankh-Morpork.
It should be noted that dwarf bread only lasts for a maximum of 300 years and belief has strange side effects.
The Scone plays an important role in The Fifth Elephant.