yes
Tarnishing is Oxidization, an extreme example is rust on iron, or the dullness of unpolished copper. The situation with Gold is that it does not combine easily with oxygen so it stays shiny. Gold...
18 carat gold should have no tarnish whatsoever, under normal circumstances. It may have some grime or dirt, which can be removed with soapy water.
Gold does not tarnish at all, unlike silver that tarnishes from H2S
No, if thick enough