metamorphic
Weathered gneiss
Foliated gneiss, chiefly quartz–feldspar–mica
AI-generated identification · may be incorrectTypically Mohs hardness 6–7, gray to pink or greenish, with a granular texture and distinct compositional banding or streaking; vitreous quartz and feldspar contrast with darker mica. Cleavage is weak overall, though mica splits readily; specific gravity is about 2.6–2.8.
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Physical properties
Typically Mohs hardness 6–7, gray to pink or greenish, with a granular texture and distinct compositional banding or streaking; vitreous quartz and feldspar contrast with darker mica. Cleavage is weak overall, though mica splits readily; specific gravity is about 2.6–2.8.
Formation & geological history
Gneiss forms when granite or other rocks undergo intense regional metamorphism at high temperature and pressure, commonly during mountain building. The image shows strongly weathered, moss-covered material; its precise age cannot be determined visually and may range from Precambrian to younger.
Uses & applications
Crushed gneiss is used for aggregate, road base, riprap, and building stone; attractive blocks and slabs may be used decoratively. Ordinary weathered pieces have little commercial value.
Geological facts
The light–dark banding reflects mineral segregation during metamorphism rather than sedimentary layering. Gneiss commonly preserves evidence of deep crustal conditions and may contain folded or stretched mineral bands.
Field identification & locations
Look for alternating light quartz–feldspar bands and dark biotite or amphibole, with visible recrystallized grains; unlike schist, it is generally coarser and less easily split. Common in ancient continental shields and metamorphic mountain belts worldwide.
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