Toning Question I

I assume you'd have noticed if other silver objects in the general vicinity were toning at the same rate (silverware, ornaments, other coins). The coin was cleaned/polished, you say? That may have "sensitized" the surface, making it tarnish faster than a mint-fresh uncleaned coin would have.

But still, a month seems pretty quick for such colouration to occur. We can rule out strange chemicals coming out of the table itself, since the side touching the table is unharmed. Was the coin sitting in the sun? Sunlight will accelerate toning; many silver compounds turn black on exposure to sunlight; leaving a cleaned coin on a windowsill for a few months is a standard method for "naturally retoning" a cleaned coin.

If not, then I'd suspect a sulfur-laden atmosphere. What kind of atmospheric environment was the coin in?
- Are there polluting industries nearby (eg. steel works)?
- Are you burning coal or heating oil such that the fumes wil be hanging around?
- Are any high-sulfur objects stored nearby (eg. rubber, eggs)?

Very odd, my guess is it's some residue from cleaning.... that once it got to the air and sunlight it became visible. But that's a wild guess and maybe not a good one.
I did see something similar (key word similar) - the wife of a friend of mine was a nurse. She pressed her uniform in the same room his coin desk was and she used spray starch which got into the air and collected on some of his coins.