This is the capstone of the Black Pyramid built by Amenemhat III, pharaoh of the Twelfth Dynasty of Egypt. He ruled from c. 1860 BC to c. 1814 BC.
The Black Pyramid was the first to house both the deceased pharaoh and his queens.
Today it still stands as a rubble mound.
What do the hieroglyphics on the Pyramid Capstone say?
“Son of Ra, Amenemhat, given life forever. King of Upper and Lower Egypt, Nimaatre, given life forever. May the face of the king of Upper and Lower Egypt, Nimaatre, be opened so that he may see the Lord of the Horizon (Horakhty, Horus of the Horizon) when he crosses the sky; may the Lord of the Horizon cause the Son of Ra, Amenemhat, to shine as a god, lord of eternity and indestructible. The Lord of the Horizon has said: ‘I have given the beautiful horizon to He of the Two Ladies (Nebty), Itjijautawy (He who inherited the Two Lands), so that you may unite with the horizon’; the horizon has said that you rest upon it, which pleases me.”