Imperial Porphyry, a famous dimension stone from Egypt

Imperial Porphyry is a pophyritic rock of trachyandesite to dacitec composition, providing U-Th-Pb zircon ages of 609-600 Ma (Abu El-Enen et al. 2007). It shows a spectacular purple color that is due to fine grained flakes of hematite, formed by pervasive hydrothermal alteration under oxidising conditions. The famous dimension stone was quarried in the Mons Porphyritis area in the Eastern Desert of Egypt from the first to the fifth century. During this period, the valuable material was processed as decorative stone for objects of art, reserved exclusively for the Imperial court of the Roman Empire. Careful polishing of the rock achieved a highly reflective finish, the characteristic purple color being regarded as an imperial status symbol. The different steps of extraction and the transport from the mountainous outcrops down to the river Nile involved masterly technical performances, all the more as a widespaced joint pattern allowed the production of large, cleavage- and fracture-free blocks, suitable for manufacturing of monolithic or sectioned columns, sarcophagy, magnificent giant bowls and sculptures. For instance, individual segments of Constantine's Column in Constantinople attain weights of up to 45 tons. Obviously, the Roman engineers had a fundamental knowledge on geological survey to detect ocurrences suitable for quarrying. After the Roman period, antique spoils of Imperial Porphyry, now known as Porfido rosso antico, were extensively re-used for sculpturing.

Abu El-Enen MM, Lorenz J, Ali KA, von Seckendorff V, Okrusch M, Schüssler U, Brätz H, Schmitt R-T (2007) Int J Earth Sci 108:2393-2408