File:Blueschist (Franciscan Complex, mid-Cretaceous; Jenner, California, USA) (16736795329).jpg

Blueschist (glaucophane schist; glaucophanite) (3.4 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.

Schist is an intermediate- to high-grade, foliated metamorphic rock. It is highly variable in appearance, depending on the mineral content, which is a function of the precursor rock and specific temperature-pressure conditions. Garden-variety schists form by metamorphism of phyllites. Schists typically have medium- to large-sized crystals, unlike the microcrystalline nature of slate & phyllite.

Schist's foliated texture (= crystals aligned into bands or stripes or layers) is often only seen when specimens are viewed on edge.

Blueschist (a.k.a. glaucophane schist; a.k.a. glaucophanite) is a scarce, glaucophane-rich rock. It is a classic example of a low-temperature, high-pressure metamorphic rock. The bluish material is the mineral glaucophane (Na2Mg3Al2Si8O22(OH)2 - sodium magnesium hydroxy-aluminosilicate). Calling this particular rock sample a “schist” is misleading, because it lacks schistose texture.