Unplated Cents
The image to the left is an example
of a 1999 D Lincoln Cent struck on
an unplated planchet.  

Since 1982 Lincoln Cents have been
struck on Copper plated zinc
planchets.  Planchets are made out
of zinc, then copper coated, and
finally struck by dies.  At times a
planchet may not get plated before
striking, or may be partially plated.  
This is considered a mint error.  

Watch out for counterfeits.  The
copper coating can be removed by
certain chemicals.  A genuine
example should have mint luster.  
Certification is
highly recommended.

Image courtesy of Joe Koelling.