benben stone

From Wikipedia:

Benben, in Egyptian mythology, or more specifically, in the Ogdoad, is the mound that arose from the primordial waters, Nu, that had upon it Atum, sitting. It was said to have turned into a small pyramid, located in Annu, which was the place Atum was said to dwell within.

The Benben stone, named after this, was a sacred stone in the solar temple of Heliopolis. It is thought to have been the prototype for later obelisks, and the capstones of the great pyramids were based on its design.