You can't compare a 1979 with a 1990... 1990 is a Copper Plated Zinc Cent The 1979 does look weird.. probably plated at one point then it wore away
My Son was very proud when he gave me that cent set. It’s one of my favorite sets. (The set is the 61 and the 69). Some coins can be both worthless and priceless at the same time! Reed. P.s. thanks for being the guy that’s helps kids do this kind of stuff in school.