Schist is a metamorphic rock consisting of mineral grains that are more or less aligned in layers. Because of this structure, schist tends to cleave into flakes or slabs.

The parent rock of a schist may be igneous (e.g., basalt, granite, syenite) or sedimentary (e.g., sandstone, mudstone, impure limestone). The metamorphic grade of a schist depends on how thoroughly melted and recrystallized its parent rock has been; higher temperatures produce lower water content, coarser crystallization, more distinct layering, and reduced schistosity. At the high end of this scale, the schists blur into the gneisses.

The directional mineral structure of schists and gneisses arises during crystallization under anisotropic stress. Anisotropic stress is stress that does not point equally in all directions, such as would be produced by placing a block of any material on a table and leaning on it at an angle. During the formation of...