I was looking into independent appraisers in WI a while ago.
Unfortunately, they are very few. Virtually all appraisials are done by jewelry stores ini Wisconsin.
The only think I found was some sort of "Midwest Gem Labs" or (something similar to that) in one of the communities just west of Milwaukee (and I remember thinking that I knew exactly where it was - to the point of a mental image of a building complex as I have had dealings with several industrial companies in the same area). Apparently they do grading for the jewelry stores in the area.
I found the listing in an internet yellow pages under "appraisers" and "Milwaukee".