It is called a "woodie". It is caused by the copper and zinc in the planchet not being properly mixed. The differing compositions then tone at different rates to different colors. Do a search for woodie or woody and you will find several on them. I am not just sure I would call them rare, but later dates are at least unusual. Small cents through the late 20's, it is rather common. Here is one of my favorite, all be it not my coin
Welcome to CT ! That is some very strong wood grain toning on that 1980. Like RLM & Thad have said, it is less common to be able to see the woodies effect on Lincoln Memorial Cents than on early Lincoln Wheat Cents. If you get a chance , check out "rlm's cent" photo gallery here on CoinTalk.
Only to us woodie addicticts But mostly when they are found in circulation because there can be such dramatically varied toning , that no 2 are identical.
If it's strong and on both sides they bring a premium. Also, if you can find a red coin with woodgraining it's certainly worth a premium.
why does this penny have a silver look to it it shows on both sides and it does not look like plating