V₀ and V_E represent the velocities, µ₀ and µ_E the refractive indices of ordinary and extraordinary rays for a doubly refracting crystal. Then

V₀ ≤ V_E , µ₀ ≥ µ_E if the crystal is calcite

In double refraction light rays always splits into two rays (O-ray & E-ray). O-ray has same velocity in all direction but E-ray has different velocity in different direction.

For calcite µ_e < µ₀ ⇒ v_e > v₀

For quartz µ_e > µ₀ ⇒ v₀ > v_e