Pleochroism is defined as the change in colour of a mineral, in plane light, on rotating the stage. It occurs when the wavelengths of the ordinary & extraordinary rays are absorbed differently on passing through a mineral, resulting in different wavelengths of light passing the mineral.

Coloured minerals, whether uniaxial or biaxial, are generally pleochroic.

To describe the pleochroism for uniaxial minerals must specify the colour which corresponds to the ordinary and extraordinary rays.

If the colour change is quite distinct the pleochroism is said to be strong.

If the colour change is minor = weak pleochroism.

For coloured uniaxial minerals, sections cut perpendicular to the c axis will show a single colour, corresponding to ordinary ray.

Sections parallel to the c crystallographic axis will exhibit the widest colour variation as both omega and epsilon are present.