As cut crystal surfaces are very rough. For any application whether it is electronic or optical, the crystal surface should be optically flat. When the surface roughness is about l/10, the surface is called optically flat (l is in the wavelength of the light for the particular application). To achieve an optically flat surface, the crystal needs to be polished in a special mounting device. There are few steps to polish the crystals. Crystals are first lapped in coarse carborundum (SiC) powder  followed by lapping in fine carborundum powder. Next comes polishing in coarse alumina emulsion (3 micron), and gradually polished in fine (0.05 micron) alumina (Al₂O₃) emulsion. This procedure results a mirror finish optically flat surface.