When such a sample is placed between two crossed polarizers, colour patterns can be observed, because polarization of a light ray is rotated after passing through a birefingent material and the amount of rotation is dependent on wavelength.
When such a sample is placed between two crossed polarizers, colour patterns can be observed due to stress induced birefringence. The reason is that polarization of a light ray is usually rotated after passing through a birefingent material and the amount of rotation is dependent on wavelength.