Birefringence (of a mineral) কী?

The birefringence of a mineral is the difference $\delta = n_{\max} - n_{\min}$ between its largest and its smallest principal refractive indices: $n_\varepsilon - n_\omega$ in absolute value for a uniaxial mineral, $n_\gamma - n_\alpha$ for a biaxial one. In a grain cut in any other direction, the difference $|n_1 - n_2|$ lies between 0 (a section perpendicular to an optic axis) and $\delta$ (a section containing the directions of $n_{\max}$ and $n_{\min}$).