The Bradt guide claims that nowhere else in the world mosaics have been used on such a massive scale, and indeed immediately upon entering, looking back above the doorway, having entered through what's called the Bab Al-Barid, you will see them all over the walls (the ceiling has been painted).

I would not dare date them, some are from restorations after the several calamities that struck the mosque. Much of it seems to be 19th-20th century renovation work, but echoing the 8th century design.