Diffraction contrast occurs due to a specific crystallographic orientation of a grain. The two phases have a tetragonal structure which consists of two sheared crystallographic unit cells. The electrical conductivity of intrinsic semiconductors can be due to crystallographic defects or electron excitation. Because grains usually have varying crystallographic orientations, grain boundaries arise. These surfaces have the symmetries of a crystallographic group. Kyanite is strongly anisotropic, in that its hardness varies depending on its crystallographic direction. There are 219 possible crystal symmetries, called crystallographic space groups. There are five 2D lattice types as given by the crystallographic restriction theorem. The toughness of corundum is sensitive to surface roughness and crystallographic orientation. A common challenge in refinement of crystal structures results from crystallographic disorder. Twinning is a phenomenon somewhere between a crystallographic defect and a grain boundary. Metallic cobalt occurs as two crystallographic structures&colon; hcp and fcc. Iron-pyrite FeS represents the prototype compound of the crystallographic pyrite structure. In materials science, crystallographic texture (or preferred orientation) can be described using Euler angles. Dislocations are present in all real crystallographic materials.