Scone of Stone
The Scone of Stone is the most famous form of dwarf bread. It has seated the Low Kings of the Dwarfs since it was first created to seat B'hrian Bloodaxe, some fifteen hundred years ago. 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.
It is noteworthy that there is a Roundworld equivalent, namely the Stone of Scone (pronounced 'scoon'), also commonly known as the Stone of Destiny or the Coronation Stone. It is a block of sandstone used for centuries in the coronation of the monarchs of Scotland, the monarchs of England, and, more recently, British monarchs. It was stolen in 1950 and returned about a year later. Even today rumours circulate that copies had been made of the Stone.
Another name the dwarfs give the Scone is the Low Throne, which is itself a parody of many a 'High Throne' such as that at Cair Paravel in the Narnia series by CS Lewis.
In Irish mythology, the lias fail (stone of destiny) is the traditional throne of the High King or Queen of All Ireland, and has a voice - it will scream or sing when the true High Monarch sits upon it. Later mythology is confused, but there is a hint that this was stolen by raiding Scots, who in turn had it stolen from them by the raiding English...
"The thing, and the whole of the thing" may also be an allusion to Kantian philosophy's ding an sich' -- the "true" thing in and of itself, which is ultimately unknowable.
The Low King's use of the Scone like a lie detector to make Dee confess is very similar to how King Arthur used Excalibur to identify a traitor in the Camelot 3000 comic series. Yes, including the way the King knew, but did not mention, something about the detection-method's efficacy.