When light is passed through a thin (typically around 50 micron
s) 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.