metamorphic
Likely gneissic amphibolite or banded metamorphic rock
Hornblende–biotite gneiss/amphibolite; exact identification requires a fresh surface or thin section
AI-generated identification · may be incorrectDark gray to black, coarse-grained, strongly foliated with elongated streaks and possible quartz/feldspar bands; likely Mohs 5–6, subvitreous to dull luster, weak cleavage along foliation, specific gravity about 2.9–3.3.
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Physical properties
Dark gray to black, coarse-grained, strongly foliated with elongated streaks and possible quartz/feldspar bands; likely Mohs 5–6, subvitreous to dull luster, weak cleavage along foliation, specific gravity about 2.9–3.3.
Formation & geological history
Formed when basaltic or mafic protoliths were subjected to regional metamorphism, recrystallizing chiefly into amphibole, plagioclase, biotite, and minor quartz; commonly Precambrian to Paleozoic in exposed metamorphic terranes.
Uses & applications
Used locally as crushed aggregate, road stone, riprap, and occasionally dimension stone; attractive polished pieces may be sold as decorative specimens, though this example appears common and unpolished.
Geological facts
The long, parallel texture records deformation and mineral alignment during mountain building. If strongly magnetic, it may contain abundant magnetite, helping distinguish it from ordinary gneiss.
Field identification & locations
Look for foliation, interlocking crystals, and dark amphibole/biotite rather than sedimentary layering; common in ancient shields and regional mountain belts worldwide, including Canada, Scandinavia, India, and the Appalachians.
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