Its could be Environmental exposure.
But it also could be something else.
How the Copper Dime Appeared
Dimes are made primarily of copper, and are coated with a mixture of copper and nickel to give the coin the appearance of a shiny silver surface. When the copper-nickel mixture wears off, the coin exposes its copper under layer, thus the brownish color of its surface.
This occurrence is considered an error, as the silver surface is not supposed to wear off. This error is called the clad coin. The non-adherence of the topmost layer of the copper dime is caused by gas and debris getting in between the layers, causing the topmost layer to separate during the lamination of the coin.
There have been instances when the topmost layer of nickel and copper mixture did not adhere on both sides of the dime, but it rarely happens. Usually only one of the sides, either the obverse or the reverse, fails to make the nickel layer adhere to the surface of the coin.
Before the dimes are distributed for circulation, they are examined if clad coins are included, and they are removed. So when a coin or two escapes from the watchful eyes of the people in the United States Mint and gets out into circulation, they are regarded rare and have higher values than what they are really worth, which is ten cents.
https://treasurepursuits.com/copper-dime/
Dont get too excited though, still only worth $5 or $6 bucks if indeed this is the case.
But i think i see some silver on that one so this may not be the case.
