Can an iron nail be used as a transformer core?

Absolutely. It would be more efficient, though, if you wrapped that nail with some stripped copper wire. You could buy an old-fashioned doorbell, take it apart, and see how it's done (small scale). When the only tool you have is a hammer, every problem begins to look like a nail. read more

As George said your nail is poor as a core. It will work, but give you troubles. Go find an old CRT computer terminal, perhaps at a recycle centre. and salvage the HV module from it. You can either modify to your needs or just use the ferrite core. Note: the frequency you use determines whether iron or ferrite. read more

The reason we use magnetic core in a transformer is to provide the magnetic field for the primary and secondary coils to step-up or step-down the voltage. There is no reason to use the non-magnetic core which would not provide the necessary magnetic field for transformer action. read more

To make a silly comparison, a steel nail (lump of steel used for attaching wood products) can be used as a core for a transformer (in very peculiar circumstances). My main intent here is to illustrate that anything from air to a nail could be used as a core for a transformer. read more