Ulexite

Ulexite is a hydrated sodium calcium borate mineral (NaCaB₅O₆(OH)₆·5H₂O) whose fibrous crystals transmit images along their length, giving it the nickname television stone. Ulexite forms as an evaporite mineral in arid-region playa lakes and borate deposits. When boron-rich water evaporates in closed desert basins, a sequence of borate minerals precipitates out, with ulexite being one of the most common.

The remarkable 'television stone' property occurs in the fibrous variety. Ulexite can grow as parallel bundles of extremely fine fibers, each acting as a natural optical fiber. When a polished slab is placed on text or an image, internal reflection within each fiber transmits the image from the bottom surface to the top, projecting it onto the surface of the stone. This is the same principle used in modern fiber-optic communication.

The cotton-ball variety grows as round, fibrous aggregates that look exactly like tufts of cotton. These form at the surface of evaporating pools where boron-rich water slowly concentrates.

The fiber-optic property is the definitive identification test. Place a polished ulexite slab on top of printed text. If the text appears projected onto the surface of the stone, it's ulexite. The cotton-ball variety is identified by its soft, white, fibrous aggregates.

Distinguish from gypsum (higher SG, different fiber structure), selenite (similar optical properties but different crystal system), and colemanite (harder, different crystal habit). Ulexite is soft enough to scratch with a fingernail.