copper nickel 2004 denver mint

I found a 2004 copper looking nickel it is a denver mint with the lewis & Clark on the back my boyfrinend thinks it might be a mistake coin. I however am scepticial, I need somthing more than a strange color to convince me. I should add that we placed the nickel on our tripple beam scale and it weighed 4.8 grm as apposed to the usual 4.9-5.0 I dont know if that is relivent or not. Also the overall apperience is good not like is coated with anything I even tried to clean it only to be left with a redish brownish older penny looking 2004 nickel. If anyone out there has any other ideas as to how to prove my boyfriend wrong for me or right I could take that too, I would be very greatfull thank you