metamorphic
Rounded dark metamorphic cobble, likely amphibolite or hornblende-rich schist
Amphibolite; principally amphibole (hornblende) and plagioclase feldspar
AI-generated identification · may be incorrectUsually Mohs 5–6, dark gray to green-black, fine- to medium-grained, with dull to weakly silky luster. Amphibolite commonly has a massive or subtly foliated texture and high specific gravity around 3.0–3.4; the specimen appears naturally water-rounded.
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Physical properties
Usually Mohs 5–6, dark gray to green-black, fine- to medium-grained, with dull to weakly silky luster. Amphibolite commonly has a massive or subtly foliated texture and high specific gravity around 3.0–3.4; the specimen appears naturally water-rounded.
Formation & geological history
Forms when basaltic or gabbroic rock undergoes medium- to high-grade regional metamorphism, commonly during mountain building. Individual grains may be hundreds of millions to billions of years old, depending on its source terrain.
Uses & applications
Used as crushed aggregate, roadstone, riprap, and occasionally dimension stone; attractive polished pieces are collected but it is not usually a jewelry gemstone.
Geological facts
Its dark color and dense feel reflect abundant iron- and magnesium-rich amphibole. Weathering may produce rusty brown staining from oxidation of iron-bearing minerals.
Field identification & locations
Look for a heavy, dark green-black cobble with faint needle-like or aligned amphibole grains; a streak is usually pale gray, and dilute acid should not fizz. Common in metamorphic belts and glacial or river gravels; visual identification is approximate without thin-section or chemical testing.
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