Green glass fragment taken from Glass Blowing Factory in Montalcino
The videos show the effect overlapping the lens above the crystal glass, moving it slowly up and down. The result shows that the refraction on the wall is more define when the lens is in a certain distance from the crystal.
Experiment with concave lens and green glass fragment given at Collevica Factory:
Experiment with concave lens and pink-purple glass fragment given at Collevica Factory: