Actually, the 'pyramid with a ring on top' is a pretty popular symbol mostly used in sci-fi movies and shows. It is always linked to the ancient egyptians (anubis, etc). If you've ever watched stargate, you might have a better idea of what I'm talking about.
Since the forerunners are on the side of being ancient beings, the pyramid with ring on top resembles them like butter on toast (in sci-fi movies and shows, it has often been used to represent their technological advancements and the spreading of their power to other planets and whatnot).
In Stargate (if you've seen the Stargate movie made way back when), the chevron with the pyramid and ring on top had many different symbolic meanings.
I'm not gonna say I'm an ancient historian or anything, but I just happen to get the idea of this symbol, or 'chevron'.
.O
/_\