The upper half of the ‘mihrab’ (prayer niche) of the Sultan Ahmet (‘Blue’) Mosque.
Partly gilt marble (recently restored). Classical Ottoman style, early 17th century.

Mihrab = a semicircular niche in the wall of a mosque that indicates the direction of the Kaaba in Mecca and hence the direction that Muslims should face when praying. The wall in which a ‘mihrab’ appears is the ‘qibla wall’.

Correspondent: J.M.Criel, Antwerpen.
Source: (amongst others) Wikipedia.