Metamorphic rocks

Metamorphic rocks are harder to define. They are any kind of rock (sedimentary, igneous, or metamorphic) that has been altered through pressure or temperature to become a new rock that is somewhat similar to the original rock, and somewhat different.

Lightly metamorphosed rocks can be very similar to the progenitor rock, whereas heavily metamorphosed rocks can bear almost no resemblance. They might have grains, or not; they might have clear crystals, or not.

Some famous metamorphic rocks include slate (slightly metamorphosed shale, which is a sedimentary rock);

http://geology.about.com/od/more_metrocks/ig/metamorphics/slate.htm

schist (medium-grade metamorphosed clays or muds), which often has shiny mica plates in it;

http://www.es.ucl.ac.uk/schools/Glossary/schist.jpg

and gneiss (highly metamorphosed granites).

http://www.anr.state.vt.us/DEC/GEO/photogallerp3.htm

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