

Generally it’s silver reacting with (trace) amounts of sulfur containing gasses in the air to form silver sulfide.
Silver sulfide isn’t blue, but the colours, and especially the multiple hues are caused by thin-film interference. That’s the same effect that gives soap bubbles that shifting multi-colour effects.
The only way to remove it, is by grinding off a tiny bit of material, which is what you do when you polish it. The tarnish doesn’t go further than the surface layer, so it’s actually a pretty amazing way to preserve the metal.
Fun fact: if you ever want to make silver look old, put it in a closed box or bag with a hard boiled (or rotten, but I suggest boiled) egg. Those are high in sulfur compounds and will add months of tarnish in minutes.





“told you I’m not gay! How dare you say I’m gay. I’ll sue you!”
Thrown in prison for being gay