Capstone

A capstone was the stone placed at the top of a pyramid.

A segment of the Key to Time was disguised as the capstone of a Martian pyramid. (AUDIO: The Judgement of Isskar)

On Terra Nova, Peri Brown, who had mutated into a half-human, half-bird hybrid, flew to the peak of a pyramid and landed just below the capstone. (PROSE: State of Change)