Optical Calcite Sphere

Optical calcite (also known as Iceland spar) refers to calcite that possesses birefringence. This is where light is refracted in two different directions, creating a double image. Hold a cube of optical calcite over an image (text works well!) and you can see the image duplicated through your crystal. There are stories of Vikings having used optical calcite to help navigate on overcast days! 

*Intuitively chosen