Wow! Antique-shop.com forums are in the big #1 spot on Google for the search term "antique shop forum", for 4 years running!Thanks to YOU.
You may (or may not!) have a coin there. I've seen some ancient Roman coins with heavy corrosion like that can clean up well enough for identification. (That's a fun activity to do with kids by the way!) My mom picked up a coin like that on the beach and gave it to me, it turned out just to be a penny from the 1980s. So where it has been can affect the development of heavy corrosion. Here is a page with instructions for cleaning ancient coins that will apply to yours no matter what the age. Let us know what you find! http://www.romancoins.net/cleaning.htm
That's a little extreme bigwull:)