What Causes Coins To Stain

There are many things that can cause a coin to stain or discolour and it is not
always possible to say exactly what.
A clear photo of both sides may help but no matter what caused it,
a stain is damage to the coin.

A coin can be improperly annealed when it leaves the mint, causing a different color (usually gray - dark gray), but its sometimes hard to tell the difference between that and environmental damage, especially on circulated coins. These are not something that you would commonly find in pocket change

Far, far more likely is that dark colors or staining are due to environmental damage after it left the mint.

Without clear, focused, and cropped pics, it's impossible to give you an accurate answer.

Coins tone for approximately ONE BILLION different reasons. Unless you toned it yourself, nobody can positively say what caused it.

For bright shiny metal objects, Earth is a rather hazardous planet. All sorts of things, in the air, in the water, in the ground, in the biosphere, can cause oxidation and corrosion of the bright shiny surface, as the environment attempts to turn the piece of metal back into the piece of ore from which it came, and this is the most common cause of "staining" of a coin. Then of course there are all the man-made chemicals a coin could theoretically be exposed to, either by accident or deliberately.

Black staining can come from numerous sources. Heat is one, especially a high-carbon flame. Simply holding a coin above a candle flame is enough to turn it black, without being hot enough to damage it. Sulfur is another cause for blackening of coins, and sulfur has a thousand everyday sources from cardboard to garlic, though this works best on silver and copper coins rather than the cupronickel face of a quarter.

Finally, it could just be ink, oil, or some other goo that's been smeared onto the coin without actually causing corrosion or reacting with the metal chemically. In which case, a good solvent like acetone can make the black "stain" come right off, revealing a perfectly normal coin underneath.