Chemical anisotropy concerns the ways in which crystals grow or dissolve in different directions. It is an appropriate subject to end this book because it brings together the oldest and the newest parts of crystal physics. Long, long ago mineralogists described the shapes of natural crystals and noted correlations with cleavage, hardness, and other physical properties. Chemical etching was another favorite topic in classical crystal physics that has undergone a recent revival because of the interest in the micromachining of semiconductor devices. Chemical anisotropy involves the interaction of a crystal with a chemically active environment that promotes dissolution or growth. For this reason it is primarily a surface property, rather than a bulk property of the crystal. This is one of the reasons why chemical anisotropy is not normally included in crystal physics books. The other reason is that rates of growth and dissolution depend on the chemical nature of the environment much more than the bulk properties of crystals do. Nevertheless, this is an important subject in contemporary crystal physics. Surfaces become more and more important as the scale of engineered devices grows smaller. The crystal physics of surface properties is a natural extension of classical crystal physics. It is a topic still in its infancy. Under favorable conditions, crystal growth takes place in such a way that the external surface is bounded by a set of plane faces. The preferred shape of rocksalt family crystals is cube bounded by six symmetry-related {100} faces. For diamond, an octahedral shape with eight {111} faces often appears. Quartz, calcite, and rutile belong to lower symmetry crystal systems with more anisotropic morphologies. Quartz crystals are often elongated along the c-axis with a hexagonal cross-section bounded by six {100} faces while the ends are terminated by six {101} and six {011} faces. Calcite tends to form rhombohedra with six faces shaped like parallelograms. Rutile (TiO2) crystals are often elongated along the c-axis forming slender needles.