When light is passed through a thin (typically around 50
microns) section of rock, the
crystal structure is such that the light is split into two
waves,
polarised vertically to each other.
On emerging from the crystal the waves display a phase difference, dependent on the thickness of the mineral and the wavelength. The two phase displaced waves interact with each other to produce the interference colour.
Unlike birefringence, this colour is not changed by rotation of the sample, but will vary in intensity.