The following is excerpted from Clifford Mishler’s Coins: Questions & Answers
Q: I have a naturally dark (not tarnished) Jefferson nickel. What caused the discoloration? Is it valuable?
A: Off-color Jefferson nickels are not uncommon, and are known in hues ranging from smoky blue through deep purple, to black. The discoloration is caused by an incorrect alloy mix containing significantly too much copper. Some collectors are attracted to them, as some prefer toned Proof coins, and will pay a small premium for them.
The discoloration of poor ally mix is more commonly encountered in the bronze cent, taking the form of coins with discernible yellow streaks on the surface or with a distinct yellowish cast. The coin is normally composed of 95 percent copper and 5 percent zinc. As the proportion of zinc to copper increases, the coins become progressively more yellow, until at a ratio of 30 to 70 percent the alloy becomes ordinary brass.