Originally posted by BarefootChessPlayer
i think the word you want is "corrosion".
the green layer on copper, when it looks nice (such as on a dome) is called "patina".
the "metal composed of a non-metal" might be metallic hydrogen. i don't know that for sure though.
I was looking for 'patina' or 'verdigris'.
Personally I would prefer to see nice shiny orange copper domes, but presumably enough people like verdigris that it's left alone; in fact it probably gets generated artificially if copper domes have to be replaced. I once went to a town where the river was exactly the colour of verdigris, quite an odd effect in conjunction with real verdigris on some of the buildings.
The metal composed of non-metal is a play-on-words.