metamorphic
Weathered amphibolite or mafic metamorphic rock
Amphibolite; chiefly hornblende amphibole and plagioclase feldspar
AI-generated identification · may be incorrectTypically dark gray-green, fine- to medium-grained, dull to weakly vitreous, with no obvious cleavage at specimen scale; hardness about 5–6 and specific gravity roughly 2.9–3.3. The rounded surface and prominent fracture appear produced by weathering and handling, though exact identification from the image is uncertain…
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Physical properties
Typically dark gray-green, fine- to medium-grained, dull to weakly vitreous, with no obvious cleavage at specimen scale; hardness about 5–6 and specific gravity roughly 2.9–3.3. The rounded surface and prominent fracture appear produced by weathering and handling, though exact identification from the image is uncertain.
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. It is commonly exposed in metamorphic terrains and stream gravels.
Uses & applications
Used as crushed stone, aggregate, riprap, and occasionally dimension stone; attractive fine-grained pieces are collected but are rarely used as gemstones.
Geological facts
Amphibolite commonly preserves metamorphosed mafic-rock chemistry while developing hornblende-rich textures. Similar-looking greenstone, basalt, or serpentinite cannot be excluded without a fresh surface and mineral testing.
Field identification & locations
Look for a dense greenish-gray rock containing black amphibole needles or grains and white plagioclase; a streak, hardness test, and magnifier help distinguish it from basalt or serpentinite. Common in ancient shield regions and mountain belts.
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