Removing sticker from a coin.

I would just leave it as is. What Condor meant by etching the surface is treating the coin with an acid prior to placing the sticker. BU coins are very shinny because they are smooth. Adhesives don't often stick well to extremely smooth surfaces. Treating the surface with an acid etch for only a few seconds would remove this smooth surface and leave a rough surface behind which is much better for a strong adhesive bond. That could be partly what you are seeing.

 

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Before hitting it with the rock tumbler, try soak/dipping it in xylene. If you don't have xylene, use cigarette lighter fluid or gasoline, but be VERY careful, toxic fumes and really flamable.

 

If you think the coin is only worth melt value now, try goo-gone before the rock tumbler

 

Goo-gone http://householdproducts.nlm.nih.gov/cgi-bin/household/brands?tbl=brands&id=13009002

 

Don't put that coin in the rock tumbler.
You'll just spread the glue over all of the rocks.

 

About rock tumblers. If you use them the other way around, they are called ball mills. When I worked in magnetic recording tape production, ball mills were still used to "grind" the magnetic paint that would be applied to the plastic film. Instead of the "balls" getting smooth from the abrasive powder, smooth balls were used to reduce the size of the chunks in the abrasive powder (which for magnetic tape was gamma iron oxide). I worked for Ampex, and some of our mills used original "balls" which were Normandy Beach flint pebbles. They turned into beautiful polished stones.