Hopefully you can see by the photo that the copper clad, above the letters "ST" in states, has separated from the zinc planchet revealing the silver colored zinc. I have seen several of these, on various dates, but only on proof coins with the 'S' mintmark. This one is a 1987, however I have seen this on other dates such as 2000, 2002, 2004, 2007, etc. What is this error called (if it qualifies as an error). I would also be curious as to the rarity and value, if any. Thanks in advance.
IMO it looks like a split plating issue. Photos of both sides of the entire coin would be appreciated. Can't help you out on the rarity and value. @Fred Weinberg, @paddyman98 and other error experts can give you more info on this.
+1 I would consider this to be extremely minor. Sometimes the zinc gets exposed because the copper is thinly plated. http://www.error-ref.com/split-peeling-plating/
Thanks for the info folks! I appreciate it. I remember, as a kid about age 10, there was a group of three of us who all collected coins. At some point, my buddy told us that any error found in a proof set was worth $1,000,000.00!!! Too bad its not that easy huh? Thanks again!
