For a certain mineral, its refractive index N is a constant, so it is one of the most important optical data and optical properties of transparent minerals. From the relevant introduction of this book, the refractive index is closely related to the optical properties, such as the luster or reflectivity of minerals. The theoretical and practical significance of the refractive index of minerals is far more than just these. It is particularly important in crystal optics and structural optical mineralogy, and is of great significance in the research and application of mineral functional materials. Here, we mainly introduce the refractive index of minerals and its application significance based on the previous research of scholars (Ye 1988; Pan 1993) and the research work of the authors of this book.