In thin sections grains appear in different textures, with different structural details, or with different “heights”. The physical reason for this appearance of relief is a difference between the refractive index of the grain compared with the refracive index of the surrounding mounting medium. If the refractive index of the grain and the mounting medium differs markedly it appears in a high relief. If the refractive indices of grain and mounting medium is about the same the grain appears in a low relief.