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