The Revealing Charm detects that no hex or charm has been placed on the coins. Of course, those that already touched and picked them up with their bare hands would have proved just as much without the spell being casted. No negative side effects have taken place as a result of that (thank Merlin). The coins were simply coins.

Cleaning away the grime makes it much easier to see the images etched onto the coins, which sparkle in the light now that they aren't covered in mud. They look old enough to date back to the Middle Ages. If you've paid visit to the chamber off the Great Hall recently, the designs on them would seem oddly familiar.

Now, the question of the hour is how did these coins get on the shore of the lake? Hmm...