metamorphic
Dark gray metamorphic rock, likely hornfels or fine-grained amphibolite
Hornfels/amphibolite; commonly quartz, feldspar, amphibole, biotite, and minor sulfides
AI-generated identification · may be incorrectHardness about 5.5–7; dark gray to black, fine-grained, dull to weakly silky luster, massive texture, and no obvious cleavage. The pale patches may be quartz or feldspar, but the photograph is insufficient for confident species-level identification.
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Physical properties
Hardness about 5.5–7; dark gray to black, fine-grained, dull to weakly silky luster, massive texture, and no obvious cleavage. The pale patches may be quartz or feldspar, but the photograph is insufficient for confident species-level identification.
Formation & geological history
Likely formed when preexisting sedimentary or volcanic rock was heated and recrystallized during contact or regional metamorphism, generally at hundreds of millions to billions of years in age depending on locality.
Uses & applications
Usually used as crushed aggregate, road stone, or durable building stone; attractive pieces may be collected, but this specimen has little gem or commercial value.
Geological facts
Its dense, tough appearance and recrystallized texture distinguish it from ordinary mudstone or basalt, though laboratory testing is needed for a firm identification.
Field identification & locations
Common in metamorphic aureoles and mountain belts worldwide. Check for reaction to dilute hydrochloric acid, streak, magnetism, grain texture, and hardness; a thin section or X-ray diffraction would confirm composition.
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