I found this 1990 penny and on the back, there is what looks like some kind of raised substance. However, I tried chipping it with my fingernail and no matter how hard I tried, nothing comes off. Unlike with other pennies where if they have glue or something on them, it comes off when I try. It kind of looks like this penny has some extra random copper layering on the back, over the Lincoln Memorial in particular. One reason I think it may be an error and not a substance from the environment is because it looks to me like some of the letters on the word Unum are raised up with the substance underneath them. Which makes me wonder if it was a mint error that happened before the words got stamped on? I guess I'm probably grasping at straws here, but what do you guys think?