Underneath the harsh toning is still a beautiful extra-fine US Walking Liberty Half Dollar. I am really starting to like overlooked beauties like these, wondering what causes the toning. One may be tempted to try to clean up a coin like this, but don't. Cleaning coins, in the Numismatic World, is a No-No! There are horror stories of the wife that cleans the husband's valuable coin collecting reducing the valuable coins into common, but shiny junk.
I have found other Mercury Dimes, like this, hiding out in the Junk Box at the Local Coin Shops, in the past. The toning can contrast the detail of the coins. Actually, I don't find many in great condition. In good natural lighting, a good eye can, sometimes, see a fantastic, beautiful rainbow of collars, hiding in the dark tone.
Read next
From @cve3