This is a question that has been bugging me for over 15 years! In high school while studying elements our teacher showed us a clear white rock or element.
It was simply flat on both sides but when placed on a sheet of paper would transfer the letters from the page beneath it to the top surface of the rock.
I remember being amazed by this rock, but could not remember what it was. Now I am working on a creative project and would love to see if I can incorporate the material in my design.