Wedge cleavage
The cleaved wedge is a cross-section of a monocrystalline substrate (+ layers), dimension about 0.6/0.6 mm, obtained by 2 or 3 cleavages along designed atomic planes that give a perfect edge, artefact free (except eventually dislocations).
Analysis of the equal thickness fringes contrast associated to a theoretical model allows for determination of semi quantitative chemical composition.