Still figuring this stuff out. The weight is what made me think that something might be wrong with it. But I just threw a few others on the scale and unless I have a whole bucket full of 1 of a kind nickels, you are very correct. Thank you for the quick reply
Nickels are composed of Cupro-Nickel. They contain copper. When exposed to the elements such as soil, sand, dirt, harsh weather and salt or brackish water for a very long time they will tone dark. Same thing will occur on clad coins which are also Cupro-Nickel. Here are many of my Metal Detected finds.. You probably found one of my thousands I have found over the years which I have dumped into Coinstar machines and was put back into circulation.
Don't worry about the weight. Most but not every single Nickel weigh 5 grams. For every kind of coin there could be a +/- in weight tolerance of a few grams. A true Nickel struck in a copper planchet intended for a Cent would be the same weight, width and length of a Cent Planchet. Here is an example from my collection..