Kind of a mix between metal and that way your hand would smell if you soaked it in a used mop bucket used to scrubdown the Superdome after Katrina. The surface of my desk at work is made of two blocks of wood, with a large crack in between them. I noticed that quite a lot of paperclips had gotten lodged in there, so I ran another paperclip in between the blocks to get them out.
A substance that can only be described as "grime" fell out onto my lap, and my hand will never be the same after brushing it off.
It was... so...horrib... so...
I don't think want to talk about it any more.