Diocletian acceded to the throne in AD 284 and, the next year, appointed Maximian to defend the western empire and restore order. This he did and was elevated to Augustus in AD 286 to serve as co-emperor, although the authority (auctoritas) of Diocletian meant that he remained first among equals.
There still was unrest, with disturbances in Germany and increased raids by Saxon and Frankish pirates on the coast of Britain and Gaul. That same year, Maximian put Carausius in command of the Roman fleet and charged him to recover control of sea. Based in Boulogne, he was successful but then was accused of allowing the raids to take place so he could recover the booty for himself. His execution was ordered, but the position of Carausius was too strong and he took refuge in Britain, where he proclaimed himself emperor. Engaged on the German frontier, Maximian could not move immediately against the usurper and, when he did, lost a naval battle in AD 289. Carausius continued to rule Britain and northern Gaul for another four years, issuing coins in which he declared himself the restorer and spirit of Britain, and legitimate colleague of the emperors, themselves.
In AD 293, Diocletian introduced the Tetrarchy, dividing imperial power among a quadrumvirate comprised of two senior emperors (Augusti) and two junior tetrarchs (Caesars). Diocletian ruled in the east and Maximian in the west, with Galerius and Constantius, respectively, to succeed them.
Constantius now was determined to recover the lost territory. Boulogne was taken, which deprived Carausius control of the Channel or access to the Continent, and he soon was murdered by Allectus, his finance minister, who ruled in his place for three more years. Finally, in AD 296, Allectus was defeated and killed when Constantius and his praetorian prefect invaded Britain.
The group illustrated above dates to about AD 300 and is situated in the Piazza San Marco, Venice. The four tetrarchs, Diocletian and Maximian, Constantius and Galerius, embrace one another, the similarity of dress and gesture asserting the solidarity of the group rather than their individual identities.
The statue itself is carved from purple porphyry, a hard purple stone quarried from Mons Porphyrites, the latter name by which it was known in antiquity (Pliny, Natural History, XXXVI.57). Pliny goes on to say that blocks of any size, however large, could be cut and that some statues of porphyry had been brought to Rome during the time of Claudius, "a novelty which was not very highly approved of, as no one has since followed his example." A surviving inscription indicates that the quarry, which was in eastern Egypt in the mountains near the Red Sea, was discovered in AD 18 during the reign of Tiberius, when it first began to be exploited. Lucan, writing later during the reign of Nero, relates that Cleopatra used porphyry (purpureusque lapis) on the walls of her palace (Pharsalia, X.116). And Nero himself was criticized for its extravagant use in his Golden House. A huge porphyry basin now in the Vatican Museums testifies to the fact. In death, his sarcophagus was of porphyry as well (Suetonius, Life of Nero, L), as was that of Helena, the mother of Constantine, which also is in the Vatican Museums. In the detail above, one can see small flecks of white and pale pink (phenocrysts) embedded in the darker stone which, says Pliny, gave porphyry its name in Greek: leptopsephos, "small pebbles."
But it was during the Tetrarchy that porphyry most frequently was used for statuary, its purple color signifying, like the purple dye of the murex shell, royalty and imperial pretension. The Edict of Diocletian (AD 301), in fact, recorded porphyry as the most precious of marbles, costing 250 denarius communis per cubic foot (XXXIII.1a; fifty of the small copper coins equaled a silver denarius).
References: Liber de Caesaribus of Sextus Aurelius Victor (1994) translated by H. W. Bird; The Brevarium Ab Urbe Condita of Eutropius (1993) translated by H. W. Bird; In Praise of Later Roman Emperors: The Panegyrici Latini (1994) translated by C. E. V. Nixon and Barbara Saylor Rodgers; "The Use and Symbolism of Polychrome Marble in Roman Sculpture" by Katie Claire McCann, in Meaning and λόγος: Proceedings from the Early Professional Interdisciplinary Conference (2013) edited by Erica Hughes, pp. 21-40.
