Spathic Calcite or Crystallized Iceland Spar.
Iceland spar is a variety of transparent and rhombohedral calcite, easily exfoliable.
It presents the optical property of birefringence, that is, it has a double refraction. According to this, if a word or line is written on a piece of paper and the mineral is passed over it, the image will appear twice.
The piece you receive may differ slightly from the photo shown as the minerals are natural and no piece is identical. The size is approximate.