Ammonium tartrate crystal
Birefringent materials polarize and split the light into two separate rays that take different paths through the crystal and travel with different speeds. When viewed between crossed polarizers (dark background) they show a colorful display of interference colors that depends on the thickness of the crystal and its orientation. The colors are enhanced by placing a magenta filter in the light path. The images show a group of ammonium tartrate crystals viewed with a polarization microscope and magenta filter.
Image by Eva Johannes, Cellular and Molecular Imaging Facility.