GIA Gem Proportion Scope. 

  A vintage diamond and gem tool. This scope uses a built in light to cast a shadow of the profile of the loose diamond or gemstone on to the included plate. The semi-translucent plate has a triple excellent cut profile of a round diamond, crown and pavilion angles printed on the plate. The loose stone is held in the wedge included at the base of the scope. Once the zoom is focused, so then the shadow is the corresponding size, the user can then compare the shadow of the profile of the stone to the angles on the printed guide to see how close or far off the stone is from an ideal cut.