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.
To me, that Euro, knowing it's age and seeing it's current condition is a small, but powerful, reminder that artifacts in the sea deteriorate at a rapid speed. And rather than leaving them in the sea until better techniques are developed for their retrieval, or waiting for specialized, elitist groups to eventually find the funds and equipment to salvage, artifacts should be brought to the surface and (supervised: ok) conservation started as soon as possible by those willing to put out their own money to do so.
Don........
The core of the Euro is steel and it was quite deteriorated. Even with the encrustation, the coin was 0.1 grams lighter than a normal Euro penny -- poof, gone.
It makes you wonder if the "cheap" coinage of today will be very hard to find 50 to 100 years from now. "They sure don't make 'em like they used to!"
I thought it might be a Euro cent but I was thinking they weighed less then a US penny. Looks almost as bad as the New Dollar coins do when they come out of the sand...