Set in the floor by the central door is the large round slab of red porphyry where King Charlemagne knelt for his coronation as emperor in the year 800 .
The porphyry in the floor is colored blue.
mnli hypothesis: The porphyry in the floor is colored blue. premise: Set in the floor by the central door is the large round slab of red porphyry where King Charlemagne knelt for his coronation as emperor in the year 800 .