Calcite exhibits a fascinating optical phenomenon known as double refraction or birefringence. When light passes through a transparent calcite crystal, it splits into two rays that travel at different speeds and take slightly different paths. This effect creates a double image when looking through the crystal, which is why calcite is sometimes called Iceland Spar in optical applications.

Additionally, calcite plays a crucial role in geology and industry. It is the primary component of limestone and marble, making it essential in construction and manufacturing. Its ability to react with acids also makes it useful in soil treatment and various chemical processes.