Born in the Purple in Rome

[riadach]

uh no it wasn't. Traditionally referred to as marble by whom?

There is purple marble and that's not the same as porphyry.

I never made such a claim. But you will find frequently in both historical and academic literature references to porphyry as marble. A quick google search of the terms porphyry, marble and Byzantium shows the same. It was frequently alluded to as Theban or Roman marble.

http://www.oum.ox.ac.uk/corsi/catalogue/classxv