I've found silver coins in the ground that you couldn't tell were dug. Ie.: there was no environmental tarnishing, etc.... It depends on the type of soil minerals, moisture, etc... Other times silver coins will be tarnished, and any attempt to remove the black/grey will render them with clues they've been "cleaned". I've even found key date lincoln cents and IH's that were numismatically redeemable. But that happens less often than silver, d/t copper is more susceptible to ground rot
And gold coins are never "ground kissed". They will look like the day they were dropped (assuming the are motionless in the ground, and not some sort of surf tossed or plowed field dings, etc...). I've found 11 gold coins, and every one was perfect. Just like gold rings, they'll still be shiny when you pull them out, even in salt, minerals, etc....
I've dug some coins and I swear you could never tell they were dug. I'm sure it goes on quite a bit, because there is a lot of burried history on this old earth of our's. You don't ask........ I won't tell
Based on metal, it's possible to determine if a coin is a dug one or not.........the signs are obvious. Silver coins for example can easily be sneaked in............and then there is the silver coin that has a condition where no one would ever think it had been in the ground, but this is based on the grade.........these are usually the ones that get dropped very early on in their life.
If you live in frozen ground territory, all coins will have some micro-scratches from frost heaving, gold, silver, even copper and nickel. But the softer ones, gold and silver will suffer more. Even some that appear nearly AU can be determined dug. Think about it, that ground has been freezing, contracting and expanding for however many years the coin was in the ground.
Had a few early dimes that looked so pristine I could call them perfect. Coin shop owner told me they were dug after looking closely and seeing these microscopic scratches that were fairly invisible to the naked eye. The damage was fairly uniform as well, demonstrating a pattern that might only be described by the frost heaving process, so very minimal though it was.