Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. The etymology of the word 'gneiss' is disputed. Some sources say it comes from the Middle High German verb gneist (to spark; so called because the rock glitters). It has occurred in English since at least 1757. Other sources claim the root to be an old Saxon mining term that seems to have meant decayed, rotten or possibly worthless material. Gneissic rocks are usually medium- to coarse-foliated and largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite...'
Contributions by Vsmith, ClueBot, and Mikenorton.