metamorphic
Dark metamorphic rock, likely hornfels or fine-grained basalt
Probable hornfels; commonly quartz–feldspar–mica with variable amphibole/pyroxene
AI-generated identification · may be incorrectDark gray, fine-grained, massive, and dull to weakly subvitreous; no obvious cleavage or crystals are visible. Hardness is approximately 5–7 Mohs, with moderate-to-high density; exact identification requires a fresh break, streak, and mineral testing.
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Physical properties
Dark gray, fine-grained, massive, and dull to weakly subvitreous; no obvious cleavage or crystals are visible. Hardness is approximately 5–7 Mohs, with moderate-to-high density; exact identification requires a fresh break, streak, and mineral testing.
Formation & geological history
If hornfels, it formed when shale or volcanic sediment was baked by a nearby intrusion, commonly during the Paleozoic–Mesozoic or younger contact-metamorphic events. A basalt alternative would be an extrusive igneous rock formed from rapidly cooled mafic lava.
Uses & applications
Suitable varieties may be used as crushed aggregate, road stone, riprap, or building stone; attractive pieces are collected but are generally not used as gemstones.
Geological facts
The surface appears weathered, with pale mineral-rich patches and possible iron staining. A hand lens may reveal tiny interlocking grains, distinguishing it from glassy slag or concrete.
Field identification & locations
Common in contact-metamorphic aureoles and volcanic terrains worldwide; test with a steel nail or knife, inspect a fresh surface, and use dilute hydrochloric acid for carbonate reaction. A petrographic microscope or chemical analysis is needed for confident classification.
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