Pure gold is soft and is not used in modern jewellery, so what we usually see are alloys containing Silver, Nickel, Copper, Zinc, Palladium, and in the case of Green gold, Cadmium. Cadmium is toxic and is no longer used, but Rose gold contains Copper, and White gold has Silver added to it. White gold also often has Rhodium plating, so you aren't even seeing the lustre of the underlying metal.
As an alloy, the metal is susceptible to reaction with environmental pollutants and off gassed chemicals from the fabric lining of the jewellery box, the glue, the wood, and any plastic.
So this is why you open your jewellery box or watch box after many months and everything is starting to look tarnished and dull.