Optical Calcite is a transparent form of Calcite which is, chemically speaking, Calcium Carbonate. Commonly called Iceland Spar thanks to Iceland’s prolific output of this crystal, this form of Calcite is eye-catching for its perfect rhombohedral cleavage and demonstration of birefringence or double refraction. Light travels through Optical Calcite in two different directions and with two different velocities (one fast, one slow) which produce two different light rays! Observe this effect by placing a piece of paper with writing on it underneath Optical Calcite to see two images slightly offset from each other. Calcite is the one of the most common minerals on earth. It occurs in a seemingly unlimited variety of shapes and colors. It constitutes a major portion of many of the earth's rocks. Calcite belongs to the calcite group of minerals, a group of related carbonates that are isomorphous with one another.