Jagger-Wolf
Allergic to metal means you're allergic to nickle.
That's a common misconception, but it's not quite on the money. Nickel is the most common metal allergy, but it's far from the
only one. Chromium and cobalt are also very common allergenic metals. Even elementally pure gold can cause contact allergies, it's just less common. (
http://www.medscape.com/viewarticle/753986 )
To reliably find out what you're actually allergic to, you have to get patch tested by a GP or immunologist.
It's not something that bars you from getting piercings, though. A lot of piercings can take glass jewellery, and high purities of titanium and niobium are common options, if you have problems with normal gold and steel.