>looks like a key stone
The vault of the Chapel of the Baptismal Font (also known as the Baptistery) within St. Peter's Basilica.
The altarpiece in mosaic was made in 1722, reproducing a design by the painter from the Marches, Carlo Maratta, which dates to 1696-1698. It represents the Baptism of Jesus by John the Baptist, in the River Jordan.