metamorphic
Likely amphibolite (hornblende gneiss or mafic metamorphic rock)
Amphibolite, chiefly hornblende amphibole and plagioclase feldspar, with possible quartz, biotite, or epidote
AI-generated identification · may be incorrectDark gray-green, coarse crystalline, and weakly foliated, with a dull to slightly glassy luster; typical hardness is about 5–6 Mohs and specific gravity 2.9–3.3. The image suggests interlocking amphibole and pale feldspar grains, though exact identification requires a fresh surface or testing.
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Physical properties
Dark gray-green, coarse crystalline, and weakly foliated, with a dull to slightly glassy luster; typical hardness is about 5–6 Mohs and specific gravity 2.9–3.3. The image suggests interlocking amphibole and pale feldspar grains, though exact identification requires a fresh surface or testing.
Formation & geological history
Formed when basalt or gabbro undergoes medium- to high-grade regional metamorphism, commonly during mountain building; ages range from Precambrian to recent metamorphic belts. Common in metamorphic terranes worldwide, including Scandinavia, Canada, the Appalachians, and parts of Africa and India.
Uses & applications
Used as crushed stone, road aggregate, riprap, railroad ballast, and occasionally dimension stone or decorative landscaping material; generally not a jewelry stone.
Geological facts
Field identification relies on its dense dark-green/black appearance, granular texture, and abundant elongated hornblende crystals; it is typically harder and heavier than ordinary basalt. A streak, hardness, and thin-section test can distinguish it from gabbro, diorite, or serpentinite.
Field identification & locations
Commonly found in metamorphic belts and near ancient mafic intrusions or volcanic rocks; look for foliation and amphibole needles, with little reaction to dilute acid. Avoid calling it definitive without petrographic or mineralogical testing.
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