Appears to be from Acid. In school chem classes we constantly experiment to see which acids do what to coins. Then the final residue is examined to see what has been formed. For example the Copper coins dipped in Sulfuric Acid will disolve some of the Copper and if removed fast enough, still be used as a coin. The resulting residue from numerous dippings produces Cupric Sulfate and if done correctly will produce a fantastic looking Clear Blue Crystal.
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