Ulexite. Sample of the mineral ulexite, a form of hydrated sodium calcium borate. It rarely forms distinct crystals, being seen instead as spongy rounded masses or the aggregate of long, hair-like fibres. This form is composed of natural optical fibres, earning it the name 'television stone'. Ulexite is formed by evaporitic precipitation in desert and semi-desert lake basins, and is a major ore of boron. Large deposits are located in the Mojave Desert (California) and in the Atacama Desert of Chile.