Calcite (CaCO3) is a naturally occurring mineral and a common constituent of sedimentary rocks. It is a birefringent crystal, which means that it exhibits birefringence, or double refraction, when light passes through it. Birefringence causes light to be separated into two polarized components, each of which travels at a different speed and direction, leading to the creation of two images. Calcite is commonly used in polarimeters and other optical devices, as well as in industrial applications, such as the production of cement and limestone. It is also used in the construction of optical instruments, such as microscopes and telescopes, because of its ability to split light into its component parts.