Raphael's famous paintingn of the "Transfiguration " (ca. 1516-20), is permanently displayed in the Vatican Pinacoteca.
This image shows a copy of that painting in fine mosaic, which was later created for St. Peter's Nave.

There are a number other works of art that have been duplicated in this way by artisans for the Basilica's nave,
The work is so fine and tesserae so small, that most visitors are unaware of the meduim used in the copies
made for the Basilica nave, or that they are actually copies of Vatican treasures.
If you look to the bottom of the picture between the candlesticks,
it is barely possible to ssee the small tesserae that make up the copy.