Imperial strength

Supporting each hemisphere are four 17–meter tall porphyry columns. Like the Romans before them, the Byzantine emperors prized porphyry marble in part because it was rare, but also because it was purple — the colour of royalty. However, porphyry is known to fracture under stress, so the basilica’s columns had to be reinforced over the centuries with bronze collars. The capitals of the columns have Justinian’s monogram carved into them.