Calcite is a common rock-forming mineral. It has many industrial uses, such as cement, and is found in many colors. It occurs in many forms of crystals, and in many types of deposits, and has a strong rhombohedral cleavage. An optically clear variety is called Iceland spar, and exhibits strong double refraction, which is when a ray of light enters the mineral, it is divided into two rays that diverge. It is too soft to be used as gem material, but many beautiful crystals have been collected the world over.