What causes this on Lincoln Cents

Hoping to get a education on what causes this on some Lincoln cents i have seen.
The silver like etching above the letter (states) partially above D (in United).
I have seen before on sides of mintmarks, number in date where there is a silver etching the same shape. I have even see on Memorial Building. Thanks in advance for sharing your knowledge.

 

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This has some good info:
http://www.lincolncentsonline.com/mechanicalDoubling.html

EDIT: Check out the part about split plate doubling.

 

Thank you very much, i could not type in the correct search words to find it , i appreciate your knowledge.

 

What furry said. You are seeing the zinc showing through the very thin copper plating.