Gneiss is a medium- to coarse-grained, semischistose metamorphic rock. It is characterized by alternating light and dark bands differing in mineral composition (coarser grained than schist). The lighter bands contain mostly quartz and feldspar, the darker often contain biotite, hornblende, garnet or graphite.

Gneiss is a type of metamorphic rock with distinct banding due to the presence of differing proportions of minerals in the various bands. Gneiss is medium- to coarse-grained and may contain abundant quartz and feldspar, which some petrographers regard as essential components.

The dark minerals sometimes exhibit an orientation determined by the pressures of metamorphism. Some specimens of gneiss contain distinctive minerals characteristic of the metamorphic environment. These minerals might include biotite, cordierite, sillimanite, kyanite, staurolite, andalusite, and garnet.

The protolith of gneiss may be an igneous rock, in this case it is called an orthogneiss. It forms probably because of shear in vicous granitic magma. that metamorphosed first into slate, then became phyllite, schist, and finally gneiss.

Migmatites have an attractive appearance, often being marked with irregular small stripes or patches of contrasting shades ranging from almost white to dark grey, and are widely used as building stone, sometimes being polished for ornament.

A migmatite is a type of metamorphic rock that was once a mixture of a silicate liquid and a restite of minerals. Migmatite means mixed rock. Most migmatites form when a solid metamorphic rock gets heated due to intrusion of magma (often granitic magma).

Garnet

metamorphic rock

Shale is a fine-grained sedimentary rock that forms from the compaction of silt and clay-size mineral particles that we commonly call “mud.” This composition places shale in a category of sedimentary rocks known as “mudstones.” Shale is distinguished from other mudstones because it is fissile and laminated.