It appears to be a leftover granite stone. I've seen several reports that the top layer was a mix of Tura limestone and granite stones.

Note that this is NOT a pristine site. There's holes drilled there in modern times, and people have been all over the thing for many thousands of years. I can't be certain, but it's not impossible that some of those stones were "adjusted" when they put up the lightning rod on top of the structure.


You can see some historic pictures here:
[curiosmos.com]

-- Byrd
Moderator, Hall of Ma'at