Quartz birefringence

Birefringence is an optical property of anisotropic materials, such as quartz, which have different refractive indices depending on the polarization direction of the light passing through them. In these materials, two distinct refractive indices are observed:

An ordinary index (n₀), for rays polarized in the plane perpendicular to the optical axis
An extraordinary index (nₑ), for rays polarized in the plane containing the optical axis

Quartz is a positive uniaxial crystal, meaning that nₑ > n₀, and its optical axis is aligned with the crystal's symmetry axis. The phase difference between the ordinary and extraordinary rays creates a phase shift.