metamorphic
Likely hornblende amphibolite fragment
Amphibolite, chiefly hornblende and plagioclase feldspar
AI-generated identification · may be incorrectDark green-black, granular to weakly foliated, with a dull to slightly vitreous luster; hornblende has Mohs hardness 5–6 and amphibolite typically has specific gravity about 2.9–3.3. The visible pale grains may be feldspar or weathered mineral inclusions.
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Physical properties
Dark green-black, granular to weakly foliated, with a dull to slightly vitreous luster; hornblende has Mohs hardness 5–6 and amphibolite typically has specific gravity about 2.9–3.3. The visible pale grains may be feldspar or weathered mineral inclusions.
Formation & geological history
Formed when basaltic or gabbroic rock undergoes medium- to high-grade regional metamorphism, commonly during mountain building; specimens may range from Precambrian to geologically young metamorphic belts.
Uses & applications
Used locally as crushed aggregate, road stone, dimension stone, and occasionally as a dark decorative rock; small unattractive fragments generally have little commercial value.
Geological facts
Its elongate dark crystals and green-black color are characteristic of amphibole-rich metamorphic rock, though a photograph cannot confirm the identification or distinguish it confidently from altered basalt, diabase, or greenstone.
Field identification & locations
Common in metamorphic terrains worldwide, including the Appalachians, Scandinavia, and many Precambrian shields. A hand lens, streak, hardness test, and thin-section or X-ray analysis are useful for confirmation.
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