Fun fact: In the Civilization II: Test of Time game, the Midgard/Fantasy scenarios have a Monolith as a "goodie hut" in the Underground level of the map. Those are brown, though, rather than stone or metal. I guess someone had a whim, and it caught on. There was a guy at one of the SF conventions I went to many years ago, who doodled up several pages of "monolith comics" and sold them in the dealers' room. I hope I still have the set I bought, because they were cute and funny. And there are a couple of comic paperbacks called "Who Was That Monolith I Saw You With?" and "Son of Monolith". It's a combination of Star Trek (TOS), the Monolith, asteroids that always get in the way, and a space dragon (also turns up in inconvenient places).