A triclinic mineral, such as vauxite, has all three of its crystal axes of different lengths, and all three interaxial angles of different sizes, with none equal to 60°, 90° or 120°. Consequently, the material is anisotropic, and physical properties, including optical properties, vary with direction.[6] The refractive index is the ratio of the speed of light in a vacuum to the speed of light through the medium.