Can anyone tell what this actually is on the S mint mark? It's a gem of a coin and hardly uncirculated and as bright as a new shield.
Looks like a plating crack to me. I've seen a lot of the post 1982 copper coated zinc pennys with cracks in the plating around the edges of designs, letters, etc. If it is a crack, it's not a die crack, but a form of post mint damage, and doesn't add any value to it.
Is it early or is this the same penny on another thread that you are questioning if it's a S over a D?
All I see is a reflection on the S that is probably playing tricks on you. The reason it looks like a gem is because it is a proof coin that was probably recently taken from a proof set. If graded, it would probably grade PR58 or PR61.