1990 steel penny with no S value?

To clear things up a bit: first, the U.S. only struck steel cents in 1943. Current cents are struck on zinc and coated with a thin layer of copper plating.

Second, the only 1990 cents to have an S mint mark are those in sealed proof set packages. All circulation cents since 1975 have only a D if struck Denver or no mint mark if from Philadelphia or West Point.

Now for the good news! Your coin could be missing its plating for one of two reasons: either it was never plated at the Mint, or someone dipped it in acid. You'd have to take it to a dealer or appraiser to be sure.

If it wasn't plated at the Mint, it's a fairly uncommon error that might retail for up to $100.

Unfortunately, if it was acid-dipped, it's only worth face value, so you will need to arrange for someone to inspect it in person.