Birefringent crystals are materials that have different refractive indices depending on the polarization and direction of light passing through them. This property causes light to split into two rays, each traveling at different speeds and angles. Common examples of birefringent crystals include calcite and quartz, which are often used in optical applications. These crystals are important in various fields, including mineralogy, optics, and photonics. Birefringence can be observed under a polarizing microscope, making it a valuable tool for identifying minerals and studying their properties. Understanding birefringent crystals helps scientists and engineers design better optical devices.