My impression is that other materials also worked. There's a brief extro in Portal 2 where Cave speaks about his experimentation with Moon Dust:
"The bean counters told me we literally could not afford to buy seven
dollars worth of moon rocks, much less seventy million. Bought 'em
anyway. Ground 'em up, mixed em into a gel."
"And guess what? Ground up moon rocks are pure poison. I am deathly
"Still, it turns out they're a great portal conductor. So now we're
gonna see if jumping in and out of these new portals can somehow leech
the lunar poison out of a man's bloodstream. When life gives you
lemons, make lemonade. [coughs] Let's all stay positive and do some
The implication is that if there are new portals (e.g. where moon dust is used as the conductor) there must have been old portals where it was not.
And to quote directly from the developer's website:
How do I let people place portals on concrete next to metal surfaces?
Any place where a concrete surface is flush with a metal surface, create a func_portal_bumper brush entity 32 units wide next to the
concrete and over the metal.
Obviously this doesn't preclude the concrete being made from moon rocks, but it seems unlikely.