Thanks to high pressures in a particular direction, the grains in some metamorphic rocks become aligned parallel to each other, giving the rock a layered appearance. Scientists call this property foliation, from the Latin word folium, which means "leaf." In slate, which is a fine-grained metamorphic rock, the foliation occurs at a small scale due to the alignment of tiny flakes of mica and other silicate materials. Geologists say slate has good cleavage because it tends to break into parallel thin sheets. In coarse-grained metamorphic rocks such as gneiss (pronounced "nice"), the foliation may occur as alternating light- and dark-colored bands. These layers may also appear wavy or curved because the forces that changed the rock acted unevenly on the rock. Rocks that metamorphose due to high temperature rather than high pressure may not show any foliation.