A

David

Darling

tourmaline

tourmaline

Tourmaline is a borosilicate mineral of variable composition (in general XY3Al6(BO3)3Si6O18(OH)4, where X = Na, Ca and Y = Al, FeIII, Li, Mg), found in pegmatites as trigonal/hexagonal crystals that are used as gemstones. Tourmaline crystals exhibit the phenomena of double refraction, dichroism, and piezoelectricity, and hence are used in polarizers and pressure-sensing devices.