metamorphic
Gneiss (likely garnet-bearing or migmatitic gneiss)
Felsic banded gneiss; chiefly quartz, feldspar, mica, and possibly garnet
AI-generated identification · may be incorrectTypically hard (about Mohs 6–7), coarse-grained, with alternating pale quartz/feldspar and dark biotite-hornblende bands; strong foliation and irregular mineral lenses are visible. The specimen has gray, white, black, and purplish-red areas, with a dull to slightly crystalline luster.
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Physical properties
Typically hard (about Mohs 6–7), coarse-grained, with alternating pale quartz/feldspar and dark biotite-hornblende bands; strong foliation and irregular mineral lenses are visible. The specimen has gray, white, black, and purplish-red areas, with a dull to slightly crystalline luster.
Formation & geological history
Formed when granite or other crustal rock underwent intense regional metamorphism at roughly feldspar-stable conditions, commonly 500–800°C and substantial pressure. Its age cannot be determined from the image; gneiss commonly records Precambrian to Paleozoic events.
Uses & applications
Used as dimension stone, paving, landscaping rock, and aggregate; attractive polished pieces are collected and occasionally used decoratively or in jewelry.
Geological facts
Gneissic banding develops from mineral segregation and deformation rather than simple layering. Purplish-red patches may represent garnet, iron staining, or altered feldspar.
Field identification & locations
Identify it by coarse crystals and compositional banding that are generally thicker and less fissile than schist; a streak, hand lens, and fresh broken surface help confirm it. Common in ancient continental shields and regional mountain belts.
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