metamorphic
Quartz–feldspar–mica metamorphic rock, likely gneiss
Gneiss; composition commonly quartz (SiO₂), feldspars, biotite/hornblende, and minor accessory minerals
AI-generated identification · may be incorrectHardness about 6–7 Mohs; mottled gray, white, black, and rusty-brown coloration with a glassy to dull luster. The specimen appears coarse-grained and weakly foliated, though the image is insufficient to confirm banding or exact mineral proportions.
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Physical properties
Hardness about 6–7 Mohs; mottled gray, white, black, and rusty-brown coloration with a glassy to dull luster. The specimen appears coarse-grained and weakly foliated, though the image is insufficient to confirm banding or exact mineral proportions.
Formation & geological history
Formed when pre-existing igneous or sedimentary rock underwent intense heat and pressure, recrystallizing without melting. Gneiss may range from Precambrian to younger ages and commonly forms in continental mountain belts.
Uses & applications
Used as dimension stone, paving, aggregate, landscaping material, and occasionally as a decorative polished stone; ordinary hand specimens have modest collector value.
Geological facts
Gneiss is recognized by segregated light and dark mineral bands or patches, produced during metamorphic recrystallization. It commonly resembles granite but shows foliation or compositional layering.
Field identification & locations
Look for interlocking quartz and feldspar grains plus alternating light and dark mineral-rich bands; a glass streak and lack of reaction to dilute acid support a silicate rock. Common in ancient shields and metamorphic mountain terrains worldwide.
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