A borosilicate mineral of variable composition (in general
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.
AND PLANETARY SCIENCE