metamorphic
Banded gneiss
Gneiss, typically quartz–feldspar–mica assemblage
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray to white, with dark biotite/hornblende bands; vitreous to pearly luster and coarse, interlocking crystals. Strong compositional banding and foliation are characteristic; cleavage is weak overall.
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Physical properties
Hardness about 6–7 Mohs; gray to white, with dark biotite/hornblende bands; vitreous to pearly luster and coarse, interlocking crystals. Strong compositional banding and foliation are characteristic; cleavage is weak overall.
Formation & geological history
Formed when preexisting granite or sedimentary rock undergoes intense heat and pressure during regional metamorphism, commonly deep in mountain belts. Individual specimens may be Precambrian to much younger, depending on locality.
Uses & applications
Used as dimension stone, paving, landscaping aggregate, and decorative architectural stone; attractive pieces are collected as geological specimens.
Geological facts
The alternating light and dark bands reflect segregation of quartz/feldspar from darker mica or amphibole during metamorphism. It may resemble banded marble, but quartz/feldspar grains remain hard and nonreactive to dilute acid.
Field identification & locations
Identify it by coarse grains, pronounced planar banding, and a granular—not sugary calcite—texture; a steel scratch test and acid test help distinguish it from marble. Common worldwide in ancient continental shields and metamorphic mountain ranges.
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