It is a type of flat or planar feature looking like the layer of the rock that is created by the alignment of the platy minerals such as mica. Thus they exist in bands of light and dark color rocks with the interlocking grains having no specific patterns.
Slate has a low metamorphic grade where the minerals are not visible with the naked eye thus usually dark-colored rock. Schist is a medium coarse-grained rock with the visible regions of mica and are often shiny due to reflections is a product of the intermediate grade.
And gneiss a course-grained rock usually of lighter and darker bands of rocks with the highest grade of foliation.
