Ferromagnetism. A 24-carat gold necklace would not be sucked into the machine for example, while a 7-carat gold necklace would (that contains steel, which contains iron). However, any kind of metal can be less dangerous for other reasons. For example, (pure, 24 Ct) gold, while not ferromagnetic, is an electrical conductor (like almost all metals), and so will heat up with the eddy currents passing through it. This could cause burn injuries if you are wearing it.