Good cleavage doesn't occur in all materials, and is displayed well in only three of the materials grown in this experiment (sodium nitrate, nickel sulfate hexahydrate, and copper acetate monohydrate). Copper acetate monohydrate was a bright blue and easy to cleave.

  The cleavage planes of sodium nitrate and calcite are paralell to the faces of the rhombohedron (as in the picture of the calcite from birefringence). There were no acceptable crystals of sodium nitrate to test this cleavage.  

Nickel sulfate cleaves along planes perpindicular to the axis of four-fold symmetry (as in the picture of nickel sulfate from physical). Showing roughly here.


 Piezoelectric Effect