Saint Peter's Cathedral in Trier, Germany. Click the image to see more detail.
It shows a sacred procession bringing relics of a saint to a church in Constantinople. The Roman emperor and empress may be seen meeting the procession at the doors of the church, surrounded by a crowd of nobles, ecclesiastics, and common people. The style of the artwork as well as the dress of the subjects seems to indicate a 5th-6th century AD date of origin, though this is the subject of debate by scholars. It is felt that the empress portrayed here could anyone from Helen, the mother of Constantine (d. AD 330) to Irene (d. AD 803).
I had never seen this particular late Roman artifact prior to yesterday. I find it to be a beautiful and evocative piece. It's a shame we have so little context for it.