What distinguishes fine from not fine? During COVID a tech asked me to get in the MRI machine wearing a face mask with a metal wire across the nose. They seemed exasperated when I refused.
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.
wise - some face masks become projectiles and are a risk to eyes. This was tested during covid. Some masks are ok. Some also have ferrous staples that attach the elastic straps, and these are also a problem.