Ulexite

Ulexite is a naturally occurring mineral with an unusual optical property. The internal fibrous structures of the rock act as naturally occurring optical fibres, transmitting light along their lengths by internal reflection. When a piece of ulexite is cut perpendicular to the orientation of the fibres and polished, it will display an image of whatever surface is adjacent to its other side. This optical property is common for synthetic fibres, but not in minerals. This strange behaviour makes the surface of the rock appear like a screen with the image of what is beneath the rock projected onto the surface, giving rise to the material’s nickname of ‘TV rock’.