Calcite and the Beginning of Optical Crystallography

An optical-quality calcite rhombohedron, Iceland spar, was a critical part of the development of optical crystallography. Today, it is a valuable tool for training future crystallographers and any person wishing to better understand the basics of polarized light microscopy. Thirty different observations can be made to demonstrate optical properties of crystals. With a little additional information about the ions that make up the crystal, those observations can identify their orientation in the crystal. A brief review of the history of the efforts to explain the puzzling double image seen when looking through a calcite rhombohedron is a nice introduction to optical crystallography.