Kamacite might be one of the classics: "Kamacite is an alloy of iron and nickel, which is found on Earth only in meteorites."[0] As a bonus, it's shiny grey,

Kamacite might be one of the classics: "Kamacite is an alloy of iron and nickel, which is found on Earth only in meteorites."[0] As a bonus, it's shiny grey, which means it could even be what this monolith is made of. [0]https://en.wikipedia.org/wiki/Kamacite https://en.wikipedia.org/wiki/Kamacite