text: Of the 32 crystallographic point groups, 10 are polar:
  The space groups associated with a polar point group do not have a discrete set of possible origin points that are unambiguously determined by symmetry elements.
  When materials having a polar point group crystal structure are heated or cooled, they may temporarily generate a voltage called pyroelectricity.
  Molecular crystals which have symmetry described by one of the polar space groups, such as sucrose, may exhibit triboluminescence.