Phyllite from the Precambrian of Wyoming, USA. (~8.8 centimeters across at its widest)

Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy.

Phyllite is a low- to intermediate-grade, foliated metamorphic rock. It forms by the metamorphism of slate. Phyllite is principally composed of microcrystalline mica and is usually light gray to dark gray to blackish in color. It has a slightly shiny luster in the light, with slightly wrinkled or crinkled foliation planes.

Stratigraphy: French Slate, upper Libby Creek Group, Snowy Range Supergroup, middle Paleoproterozoic depositional date (probably 2.0 to 2.06 Ga); late Paleoproterozoic metamorphic date (1.76 to 1.78 Ga)