metamorphic

Metamorphosed mafic rock, likely amphibolite

Amphibolite, chiefly hornblende + plagioclase; possibly mafic gneiss or altered basalt

AI-generated identification · may be incorrect

Dark gray, fine- to medium-grained, weakly foliated with pale mineral bands and fractures; likely hardness 5–6, vitreous to dull luster, granular crystalline texture, and poor cleavage overall. Exact identification requires thin-section or mineral testing.

Identified September 15, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when basalt or gabbro was metamorphosed under amphibolite-facies heat and pressure, commonly during mountain building; age cannot be determined from the image. Typical parent rocks range from Precambrian to younger tectonic terranes.

Uses & applications

Used locally as crushed aggregate, road stone, dimension stone, and occasionally architectural facing; attractive specimens are collected but are generally not gem material.

Geological facts

Amphibolite commonly contains black hornblende and white-gray plagioclase, producing a salt-and-pepper appearance. Brown weathering may reflect iron oxidation or biotite alteration.

Field identification & locations

Look for dense dark rock, aligned amphibole grains, and white plagioclase streaks; it is commonly found in metamorphic belts and basement exposures. A magnet, streak, acid, and hardness tests can help distinguish it from basalt, slate, and gneiss.