top two are the 2016 dime and the bottom two are the 1991 the 1991 has a weird mark arching from his forehead to his hair
it's most likely toning because it isn't rust. just thought they were cool looking so I wanted to show.
yes...crimping damage/‘91...but consider this on the ‘16... yes, it is toning, but, imho, it is too new a date to be natural...my guess is someone exposed it to some kind of gas in an experiment in AT...just an observation...Spark