The best-known birefringent crystal is the mineral calcite (Iceland spar), the colorless, transparent rhombohedral salt calcium carbonate, . The optical axis coincides with the three-fold axes of the equilateral groups, along which the ions are also situated. For 590 nm light, the yellow sodium-D lines, the indices for calcite are =1.658, =1.486, =0.172. Some other common uniaxial minerals are: tourmaline, beryl, quartz, ruby, sapphire and zircon. Birefringent materials find several applications in optics, for example Nicol prisms and quarter-wave plates.

In this Demonstration, the angle of incidence of a light ray entering a crystal can be varied, as can the two refractive indices (the default values are those of calcite). The direction of the optical axis is marked with a gray arrow. The ordinary ray is shown in orange, the extraordinary ray in blue. After refraction by the crystal, the light emerges in two parallel rays. You also have the option to display the polarizations of the various rays.