Ulexite (/juːˈlɛksaɪt/) sometimes called TV stone due to its unusual optical properties, is a hydrous borate hydroxide of sodium and calcium with the chemical formula NaCaB5O6(OH)6·5H2O. The natural fibers of ulexite act as optical fibers, transmitting light along their long axes by internal reflection. When a piece of ulexite 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. 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.

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