metamorphic
Dark mafic metamorphic rock, likely amphibolite
Amphibolite; chiefly hornblende amphibole and plagioclase feldspar
AI-generated identification · may be incorrectTypically dark green-black, coarse to fine grained, with a dull to weakly vitreous luster; Mohs hardness about 5–6, nonporous, and lacking obvious cleavage at the hand-specimen scale. The image suggests a foliated, weathered mafic rock, but mineral identification is uncertain without testing.
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Physical properties
Typically dark green-black, coarse to fine grained, with a dull to weakly vitreous luster; Mohs hardness about 5–6, nonporous, and lacking obvious cleavage at the hand-specimen scale. The image suggests a foliated, weathered mafic rock, but mineral identification is uncertain without testing.
Formation & geological history
Forms when basalt or gabbro undergoes medium- to high-grade regional metamorphism, commonly during mountain building; ages range from Precambrian to modern tectonic belts. Commonly occurs in metamorphic terranes and amphibolite-facies zones.
Uses & applications
Used locally as crushed stone, aggregate, road material, and occasionally dimension stone; attractive polished pieces are collected but it is not generally a jewelry gemstone.
Geological facts
Its dark color reflects abundant iron- and magnesium-rich amphibole and commonly plagioclase. A greenish-black appearance and possible foliation distinguish it from many lighter metamorphic rocks.
Field identification & locations
Look for dense, dark rock with interlocking amphibole grains and faint alignment; a streak, hardness, magnetism, and thin-section or X-ray testing improve identification. Common in ancient continental shields and metamorphic mountain belts.
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