Which image? Marble is a softer stone than agate. And agate (or quartz family minerals) are the typical stones of intaglio and cameo carving—particularly at this period.

Among the mosaics I showed, from Sicily, occasionally a red stone called "porphyry" is used. It is on the slightly dark and violet end of red, and tends to be spotty. Some sources say porphyry is a "marble." Other insist it's "feldspar" or "quartz." My impression is that the stone if antiquity is softer than quartz. Lots of people used porphyry for decorative stonework. At Egypt, whole sarcophagi and big statues were carved from it.

Jamey