[nahys] /naɪs/

a metamorphic rock, generally made up of bands that differ in color and composition, some bands being rich in feldspar and quartz, others rich in hornblende or mica.
any coarse-grained metamorphic rock that is banded and foliated: represents the last stage in the metamorphism of rocks before melting

1757, from German Gneiss “type of metamorphic rock,” probably from Middle High German gneist “spark” (so called because the rock glitters), from Old High German gneisto “spark” (cf. Old English gnast “spark,” Old Norse gneisti).
A highly foliated, coarse-grained metamorphic rock consisting of light-colored layers, usually of quartz and feldspar, alternating with dark-colored layers of other minerals, usually hornblende and biotite. Individual grains are often visible between layers. Gneiss forms as the result of the regional metamorphism of igneous, sedimentary, or other metamorphic rocks.


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