igneous
Dark coarse-grained rock, likely gabbro or amphibolite
Gabbro (plutonic mafic rock; mainly plagioclase and pyroxene), possibly amphibolite if metamorphosed
AI-generated identification · may be incorrectBlack to dark gray, granular texture with weak vitreous-to-dull luster; likely Mohs hardness 5–6, no obvious cleavage as a whole rock, and specific gravity about 2.8–3.2. Individual dark minerals may show cleavage, while pale grains are commonly plagioclase.
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Physical properties
Black to dark gray, granular texture with weak vitreous-to-dull luster; likely Mohs hardness 5–6, no obvious cleavage as a whole rock, and specific gravity about 2.8–3.2. Individual dark minerals may show cleavage, while pale grains are commonly plagioclase.
Formation & geological history
Likely crystallized slowly from mafic magma deep in the crust, generally Precambrian to younger; alternatively, basaltic protolith recrystallized during metamorphism to form amphibolite. The photo alone cannot establish age or exact identity.
Uses & applications
Used as crushed aggregate, road stone, riprap, dimension stone, and occasionally decorative material; not typically a jewelry gemstone. Collector value is usually low unless unusually fresh, textured, or locality-specific.
Geological facts
Gabbro is the coarse-grained intrusive equivalent of basalt. Amphibolite commonly contains hornblende and may display a foliated or schistose texture.
Field identification & locations
Common in oceanic crust, layered intrusions, and metamorphic belts worldwide. Identify by its heavy, dark, interlocking grains; a hand lens, streak, magnet test, and thin-section or chemical analysis would distinguish gabbro from amphibolite.
