Calcite birefringence seen though a linear polarization filter to remove one of the images. Calcite crystal (calcium carbonate, CaCO3) showing strong birefringence (double refraction). This transparent variety called Iceland spar is used for optical purposes. The strong birefringence causes objects viewed through a clear piece of calcite to appear doubled. The birefringent effect (using calcite) was first described by the Danish scientist Rasmus Bartholin in 1669. The double image is really made up of two images each with a different polarization orientation. The images can be separated with the use of a polarization filter.