The current color on your coin is normal oxidation due to the alloy of that coin type. Since you found it while detecting, I am a bit surprised it has no visible pitting. Unfortunately, there is no practical way of returning the coin to a like-new luster. It is worth face value. If you want to find some quality John Adams dollars on the cheap, just ask the tellers. While most rolls will be a mix, there may be a few solid rolls of John Adams near you.
Just an aside, but when roll hunting, if what I want (typically halves and nickels) are not available, I will take whatever denomination the bank has available. I have found several errors among the small dollar coins. Cracked dies, cuds, machine doubling, and the like. Since I do not collect small dollars, these become available for trade with other collectors.