Here is a fairly close shot of the mosaic detail in one of the domes of the inner narthex at the Chora Church (museum). The original church, built in the 5th century, survived a number of disasters, both natural and man-made. The Byzantine mosaics date from the early 14th century. More about the church here.
This old face is in the Great Palace Mosaic Museum (Büyük Saray Mozaikleri Müzesi) in Istanbul. The mosaics contained in this museum were unearthed only recently (1930s and 1950s). These mosaics are likely from the 5th century, and were part of the Great Palace of Constantinople. More images here and here.