metamorphic
Banded amphibolite (likely hornblende schist)
Amphibolite; chiefly hornblende ± actinolite and plagioclase feldspar
AI-generated identification · may be incorrectTypically dark green to black with gray-green bands, vitreous to dull luster, and a foliated or lineated texture. Hardness is about 5–6; cleavage is weak overall but amphibole crystals show good cleavage, and specific gravity is roughly 2.9–3.4.
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Physical properties
Typically dark green to black with gray-green bands, vitreous to dull luster, and a foliated or lineated texture. Hardness is about 5–6; cleavage is weak overall but amphibole crystals show good cleavage, and specific gravity is roughly 2.9–3.4.
Formation & geological history
Formed when basaltic or gabbroic protoliths underwent medium- to high-grade regional metamorphism, commonly during mountain building. The photographed specimen may preserve compositional layering rather than true sedimentary bedding; its exact age cannot be determined from appearance alone.
Uses & applications
Used locally as crushed stone, aggregate, dimension stone, and occasionally architectural facing. Attractive polished pieces are collected, though it is uncommon in fine jewelry.
Geological facts
Amphibolite’s green-black color reflects iron- and magnesium-rich amphibole, while pale bands commonly contain plagioclase. It can resemble greenstone, serpentinite, or banded gneiss.
Field identification & locations
Look for strong foliation, elongated dark amphibole grains, pale feldspar-rich bands, and a heavy, nonporous feel; a streak, hardness, and thin-section test improve confirmation. Common in metamorphic belts worldwide, including the Appalachians, Canadian Shield, and Precambrian terrains.
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