Looks like natural silver tarnish

Gotcha, I've seen some of these coins with that tarnish and others without.  I'm wondering why that would be but suspect the answer isn't easy to come across.

I actually photographed mine and stuck pics on here when it arrived.

https://bt.irlbtc.com/view/903503.msg10356400#msg10356400

These are from being just dug out. It's definitely deteriorated.

sorry to hijack btw,

here is a pic of mine, basically the same as shown above

you should never clean the coin (obviously like any rare coin), but from talking to antique coin collectors i understand the tarnish/patina is reversible.

Thanks for the info everyone.  Even if I don't sell them, at least I know to get them out of the cases they are currently in.     Anyone know where to buy airtights that will fit these coins?  I'm assuming they are the same dimensions as the original silver Casascius coins, but I don't have any of those so I don't know for sure. 

Hmmm, I should check up on my Bitcoin Bowl coin and see if it has any tarnish. If not I should probably get it graded ASAP or at least move it into an airtite. I didn't know that the original holder would cause this issue (which is quite annoying IMO).

I handle coins with bare hands all of the time. You can safely hold them by the rim with a few steps beforehand.
Step 1 Wash your hands really well.
Step 2 Only hold them over a soft towel - I have dropped several Cas coins with no damage and yes they still graded well.

Cotton gloves are bad to use - they leave little lint balls and give you a better chance of dropping the coin.
Never read up on nitrile gloves...I have seen some people here using them.

When I crack a slab or open an air-tite I just left the coin fall onto a microfiber towel...then grab it with my hands. Coins are strong and do not ding easy it is really just finger prints that cause the real damage or wiping them.

They are fairly well known to most Casascius collectors and they do not have a hologram or private keys. I think it is the only silver coin Mike has made since he was prevented from selling loaded ones. The mintage of these St Petersburg coins is 2,000 - however over half of these were scrapped because he was not happy with the quality.