Each of tapering pentagonal out-shape, with a rocaille-cast frame cornered by caryatid figures and set to each side with a porphyry panel, on scrolled foliate feet.

Porphyry was originally used in the ancient Egyptian Empire taken from the Mons Porphyrites, at the time the only known mining area for porphyry. Porphyry became extremely popular amongst the Romans during the Roman Tetrarchy (late 3rd century) and its purple colour was so admired that its use was reserved solely for the Emperors. Constantine used circles of porphyry for the floors of his palaces, in rooms only he was allowed to enter and into which his sons were born, and the stone was also used in the construction of important burial sarcophagi. The rare stone has ever since been prized as a very important and desirable material and was later used in the decoration of St. Peter's Basilica and for the construction of important crowned busts.