I have a slightly odd enquiry. I'm an art historian trying to identify some marbles painted in a fresco of the Last Supper of the mid fifteenth century (by Andrea del Castagno at Sant' Apollonia, Florence) . The images are attached. It's the marble rebates above the apostles' heads. Of course, they might be in part invented, but I'm fairly sure he was trying to depict serpentine and porphery on the left. It's the remaining marbles that have me confused, particularly the one which is voilently veined in the centre above the head of Judas. Any help would be much appreciated.