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.
I used F'x answer as a starting point, and I found the mineral. It's called Ulexite.
The fibers of ulexite act as optical fibers, transmitting light along their lengths by internal reflection. When a piece of ulexite is cut with flat polished faces perpendicular to the orientation of the fibers, a good-quality specimen will display an image of whatever surface is adjacent to its other side (as shown in the photograph).
I'm not sure I understand what you want from the description above “transfer the letters”, but there is a typical high-school demonstration that I can think of that closely matches this description: that of birefringence.
Calcite (featured above) was the first material with which this effect was observed, in 1669, and still an easily obtainable material for demonstrating it.
