Strong parallel alignment of different mineral bands in a rock is called foliation. Foliation occurs in metamorphic rocks. The high pressure under which metamorphic rocks are formed creates this banding.
Foliated metamorphic rocks formed at low temperatures and pressures are called slate. Slate breaks along nearly perfect parallel lines. Phyllite forms at low to intermediate temperatures and pressures. Layers in phyllite are more crinkly than the ones in slate, and these rocks contain small crystals .At intermediate to high temperatures and pressures, schist forms. Schist contains larger crystals, and the grains are aligned in parallel to subparallel layers. Gneiss exhibits alternating dark and light bands of minerals. It is formed at very high temperatures and pressures.