Well, I decided to try my hand at cleaning.
I soaked the coin for about 12 minutes in Hydrogen Peroxide then used a dental tool to try and pick of the encrustation. It was a slow slog... soak, pick, repeat.
After several cycles I saw a star appear under one edge then the letter "E" -- very clear both. This gave me hope and I redoubled my efforts this time using an ultrasonic vibration to help the process along. Now more letters appeared. I got the distinct feeling that this was not an American coin but was still unable to discern the words or symbols that were starting to appear.
I used a powerful light and strained my eyes to their breaking point and finally was able to see the following:
"E" "I" "N" "E" "U" "R" "O" "C" "E" "N" "T"
I had to laugh. My priceless, mysterious, ancient coin, found near a known treasure site -- that No One (except for LaZoOro) had insight on -- turned out to be a modern Euro penny!
LaZoOro was correct, it was modern and a penny -- but was a long way from home.
I learned a lot and had fun. Going forward, I am going to be much more careful if using a dental pick; it easily sratched the copper sheathing.