Does a Crystal Radio coil require enameled wire or insulated turns?
If you use insulated wire, the current is forced to go all the way around each loop, and this helps add to the magnetic field that is crucial for the operation of an inductor. More turns = more field and more inductance.

If you use bare wire, the current will take the least resistance path, which would be the shortest path from one end to the other, and there will be virtually no magnetic field at all.