Metamorphic rocks are formed when existing rocks are subjected to extreme heat and pressure. The four most common types of metamorphic rocks are SLATE, gneiss, SCHIST, and MARBLE. SLATE is a fine-grained rock that is usually gray or black in color. Gneiss is a coarse-grained rock that is characterized by alternating light and dark bands. SCHIST is a medium-grained rock that has a layered or foliated texture. MARBLE is a fine-grained rock that is usually white or light in color.