metamorphic
Banded gneiss pebble
Foliated gneiss, typically quartz–feldspar–mica bearing
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray-brown with distinctive wavy light and dark bands, dull to slightly silky luster, and strong foliation. Composed mainly of quartz, feldspar, and biotite or other mica; cleavage is weak overall, though mica layers may split.
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Physical properties
Hardness about 6–7 Mohs; gray-brown with distinctive wavy light and dark bands, dull to slightly silky luster, and strong foliation. Composed mainly of quartz, feldspar, and biotite or other mica; cleavage is weak overall, though mica layers may split.
Formation & geological history
Formed when pre-existing granite or sedimentary rock underwent intense regional metamorphism, causing mineral segregation and banding. Most examples are Precambrian to Paleozoic, though gneiss forms in many ancient continental terrains.
Uses & applications
Used as crushed aggregate, dimension stone, paving, landscaping rock, and decorative slabs; attractive polished pieces are collected as geological specimens.
Geological facts
The banding records extreme heat and pressure deep in the crust, often during mountain building. Rounded edges indicate transport and abrasion in a river, beach, or glacial environment.
Field identification & locations
Identify it by alternating pale quartz-feldspar bands and darker mica-rich streaks, unlike uniformly colored granite. Common worldwide in ancient shield regions and as glacial or stream cobbles; a hand lens can reveal mica flakes.
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