What is the name of the capstone that topped obelisks?

The capstone that topped Egyptian obelisks was called a pyramidion. The same term also describes the pointed capstone of an Egyptian pyramid, so it refers to the small pyramidal element crowning either kind of monument.

Pyramidia were usually made from stone such as limestone, sandstone, basalt, or granite. Some were covered with copper, gold, or electrum, reflecting the ancient Egyptian association between the pyramid shape and the sun’s rays. From the Middle Kingdom onward, many carried royal names, titles, or religious scenes.

A common mix-up is benben: the Benben was the sacred primordial stone associated with creation at Heliopolis, while benbenet was an Egyptian term connected with pyramidia. Naos means a shrine or sacred container, and mastaba describes a flat-roofed tomb, so neither is a capstone.

Obelisk pyramidia were often integral to the monument’s pointed upper section rather than separate objects. A later famous example is the aluminum pyramidion placed atop the Washington Monument in 1876.