A ground rim is perfectly flat and may have tiny little chips along the edge when looked at though a lens. It may also be less than shiny. A unground rim, i.e., one straight out of the mould, is slightly rounded and shiny. It can help to give an idea of relative age; the more hand finishing the older the item, i.e., less automation. The two clear rims are on identical plates, except one has a ground rim and the other doesn't (Oh and the older one has its name stamped on it, so a different plunger as well). The green one has a ground rim as well.