Why the Mint tests planchet hardness, what a genuine test mark looks like on a finished cent, and what is only damage.
Diagnostics of Fake and Genuine Unplated Zinc Cents
in Cents, Planchet errors
How to tell a genuinely unplated zinc cent from one that has been corroded, stripped by reverse electrolysis or re-plated — and the surface clues that give each away.