Uniaxial crystal
Uniaxial crystals are transmissive optical elements in which the refractive index of one crystal axis is different from the other two crystal axes (i.e. "n" ≠ "n" = "n"). This unique axis is called the extraordinary axis and is also referred to as the optic axis. Light travels with a higher phase velocity through an axis that has the smallest refractive index and this axis is called the fast axis. Similarly, an axis which has the highest refractive index is called a slow axis since the phase velocity of light is the lowest along this axis. These crystals  exhibit a type of birefringence. Negative and positive types. The optic axis can be the fast or the slow axis for the crystal depending upon the material. Negative uniaxial crystals (e.g. calcite CaCO, ruby AlO) have "n" < "n" so for these crystals, the extraordinary axis (optic axis) is the fast axis whereas for positive uniaxial crystals (e.g. quartz SiO, sellaite (magnesium fluoride) MgF, rutile TiO), "n" > "n " and thus the extraordinary axis (optic axis) is the slow axis.