Funny you asked that question today! I'm in the middle of researching an article about the use of nickel in jewelry and nickel allergies
What I have learned is that nickel is common in gold and silver plated jewelry. The piece of metal that is plated usually starts out as a base metal, such as brass. A nickel flash is applied to the surface to help the silver or gold plating adhere to the brass.
For the time being, I can tell you that 12k yellow gold and higher is nickel free. White gold usually contains some nickel, so I would stay away from that and plated jewelry for now. I will keep researching and publish the article soon.