When I was in Pforzheim, Germany I visited the town museum.
Considering the size of the city, it has an incredible collection of
jewelry from early historical examples to contemporary German
artists. I loved the contemporary work but unfortunately it was a
smaller proportion of the work than I would have liked . I believe
the school there has a museum too, though I didn't get a chance to
visit the school. If your looking for an example of a museum
featuring stones, you can't go very wrong checking out the museum in
Ider-Oberstein, sorry I can't remember the name of the museum, but
it's on the main road going into town and not hard to miss.