Metamorphic rocks have been modified by heat, pressure and chemical processes, usually while buried deep below the Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture and chemical composition of the rocks.
There are two basic types of metamorphic rocks:
Foliated metamorphic rocks such as gneiss, phyllite, schist and slate have a layered or banded appearance that is produced by exposure to heat and directed pressure.
Non-foliated metamorphic rocks such as hornfels, marble, quartzite and novaculite do not have a layered or banded appearance.
Sources: https://geology.com/rocks/metamorphic-rocks.shtml
