One common type was made from an oxidized razor blade (rusty or flamed) with a pencil lead pressed against the blade with a safety pin. The oxide layer on the blade and the point contact of the pencil lead form a semiconductor Schottky diode and only allow current to pass in one direction.
Yep I mentioned that in a comment below, an accidental semiconductor IS possible, but that wasn't my point really. I get the arguers point though, a headboard implies accidental, I wasn't thinking of it that way. But anyway, as we've established, both improvised and accidental are possible and documented.
