metamorphic
Dark coarse-grained metamorphic rock, likely amphibolite or hornblende schist
Amphibolite, dominated by hornblende amphibole and plagioclase feldspar; exact identification requires testing
AI-generated identification · may be incorrectDark gray-brown, granular to weakly foliated, with sparkly cleavage surfaces and no obvious glassy luster. Typical amphibolite hardness is about 5–6 Mohs, with blocky fracture, imperfect foliation, and specific gravity around 2.9–3.2.
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Physical properties
Dark gray-brown, granular to weakly foliated, with sparkly cleavage surfaces and no obvious glassy luster. Typical amphibolite hardness is about 5–6 Mohs, with blocky fracture, imperfect foliation, and specific gravity around 2.9–3.2.
Formation & geological history
Formed when basaltic or gabbroic rock underwent medium- to high-grade regional metamorphism, commonly during mountain building; age depends on its locality and cannot be determined from the image.
Uses & applications
Used locally as crushed aggregate, road stone, riprap, and occasionally dimension stone; generally has little jewelry value because it is opaque and not easily polished.
Geological facts
The dark minerals are likely amphiboles, which commonly produce the rock’s elongated or glittering texture. A precise distinction from hornblende schist, diabase, or dark gneiss requires a fresh surface and mineral tests.
Field identification & locations
Look for abundant black amphibole, gray feldspar, moderate weight, and a nonreactive surface with dilute acid; a streak, hardness, magnetism, and thin-section analysis would improve identification. Common in metamorphic belts worldwide, including areas formerly containing volcanic rocks.
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