Ulexite is called TV Rock because of it unusual optical properties. It acts like a natural fiber optic mineral!
Named after the 19th century German chemist, G. L. Ulex, who first discovered it, ulexite is a mineral occurring in silky white rounded crystalline masses or in parallel fibers. It is found with the mineral borax, directly deposited in arid regions from the evaporation of water in intermittent lakes. It is usually found as a kind of "cotton ball" tuft of crystals, but sometimes appears in a vein-like bedding of closely-packed, fibrous crystals. This second form of the mineral shows the behavior that caused it to be known as TV rock.
The fiber optic properties of ulexite probably come from its birefringence along with the packing of parallel fibers. Because of its birefringence, different polarizations of light travel along the fibers in differing directions. The "slow" axis of the crystal is along the fiber, causing one polarization of light to be channeled along by total internal reflection. The other polarization, responding to the "fast" axis, will tend to leak from one crystal fiber into an adjacent one - where it will enter aligned for the "slow" axis and be channeled.
You can see the birefringence of ulexite (actually, it is called biaxial birefringence) by shining a laser onto one face of TV rock at an angle to the surface. The light will emerge in two or three beams with differing polarizations. (If you happen to have a polarizer handy, you can check this!)
Specimens are about 1.5" in size, with two polished faces.