just throwing in the word "amber".
its quite famous, has a mysterious piece of art linked to it (http://en.wikipedia.org/wiki/Amber_Room) and his own early-on trade route history (http://en.wikipedia.org/wiki/Amber_Road).
Plus it could be used by some baltic civ to be included (lithuania for instance).
Not realy likely but it would have some potential
Amber doesn't give away the identity of a Civ though.