Ulexite (NaCaB5O9·8H2O) (hydrated sodium calcium borate hydroxide) is a mineral occurring in silky white rounded crystalline masses or in parallel fibers. It was named after the 19th century German chemist G. L. Ulex who first discovered it.

Ulexite is a structurally complex mineral, with a basic structure containing chains of sodium, water and hydroxide octahedra. The chains are linked together by calcium, water, hydroxide and oxygen polyhedra and massive boron units. The boron units have a formula of B5O6(OH)6 and a charge of -3, and are composed of three borate tetrahedra and two borate triangular groups. Hardness is 2 (softer than a fingernail) and specific gravity is approximately 1.97.

Ulexite is found with the mineral borax and is directly deposited in arid regions from the evaporation of water in intermittent lakes called playas. The precipitated ulexite commonly forms a "cotton ball" tuft of acicular crystals. Ulexite is also found in a vein-like bedding habit composed of closely-packed fibrous crystals, also known as "TV rock" or "TV stone" due to its unusual optical characteristics. The fibers of TV rock act as fiber optics, transmitting light along their lengths by internal reflection, and when a piece of TV rock is cut with flat polished faces perpendicular to the orientation of the fibers a good-quality specimen will display an image of whatever surface is adjacent to its other side (as shown in the photograph).

The fiber-optic effect is the result of the polarization of light into slow and fast rays within each fiber, the internal reflection of the slow ray, and the refraction of the fast ray into the slow ray of an adjacent fiber. An interesting consequence is the generation of three cones, two of which are polarized, when a laser beam obliquely illuminates the fibers. These cones can be seen when viewing a light source through the mineral as discovered by E. Aalto and explained by D. Garlick and B. Kamb (J.Geol.Ed., 1991).

Ulexite decomposes in hot water.

Ulexite is found principally in California and Nevada, USA; Tarapaca Region in Chile, and Kazakhstan.