I have always wondered what can cause some of the discolarations on the copper cent. I have seen black, dark red (not paint), gold toned, yellowish, green and I have even seen a 1920's penny that looks to be silver and black colored. These are not painted either.
I'm not sure what causes the exact colors to appear, but I do know that they come from chemicals and gases in the environments around the coins. Coins kept in the cardboard holder books will tone from the acids in the cardboard holding them... Hopefully thats a starting point for what you are looking for. And for the more experienced among feel free to fix what I didn't quite get right....
It would help if you were to post a pic. From your description, it sounds like a toned wood grained coin. The wood graining is caused by improperly mixed alloy and the toning is usually caused by sulfur reacting with the copper. I have an album of woodies at my home page or for the ultimate, see http://www.cointalk.com/forum/t48345/#post527224.
This is a great question....how about from oils and grease on the hands of the thousands of people that have held them. Does this play a role in it?
Copper is one of the most reactive metals. It has an ability to chemically bond with virtually any anionic substance (negatively charged). Based on my research, sulfur (also very reactive) is thought to be responsible for most of the toning. When bonded to a metal on a microscopic level, it can refract light to produce a variety of colors, depending on the molecular depth and likey also depending on what other elements are bound to the surface.
Badthad really goes in depth...but from what i recall from science classes, when we see the colour of things, what is occuring is all the colours of the spectrum are obsorbed except the colour we see the item as...that colour is refracted back into our eyes. Have i got it right ?
All the above! Althrough environment has a lot to do with how a cent will tone, the alloy of the cent will be a bigger factor. Over the years the cent has been made with different metal compounds, copper, brass, and bronze which will react differentially to the environment. The 1909 Copper/Tin/Zinc (Bronze) cent will react differentially then the 1944 Copper/Zinc (Brass) cent and likewise to the 1982 Copper Plates Zinc cent. Mainly a cent will be classified as either red, brown, or red/brown but in the right conditions a cent can change to a lot of colors.
