There is as well an interesting use of traditional Christian symbols, for instance this is the famous depiction of the seraphim with six wings, covering up. This same depiction can still be seen in Hagia Sophia. It is one of the few Christian symbols that were not defaced by the Muslims when they turned it into a mosque.
And there are some interesting mosaics in boxes in various places. This is surely the only one in the world that features both the black Madonna of Russian Orthodoxy, and the Statue of Liberty!
Here is a shot of the beautiful bas-relief of the Holy Spirit being dispensed at Pentecost to all sorts of persons, including, prominently Mary (see Acts 1.14). This is located beneath the organ in the back of the basilica.