metamorphic
Banded amphibolite (hornblende-rich metamorphic rock)
Amphibolite; predominantly hornblende ± plagioclase, with possible biotite and quartz
AI-generated identification · may be incorrectDark greenish-black, fine- to medium-grained, weakly foliated and elongated, with a dull to subtly vitreous luster; typical hardness is about 5–6 Mohs and specific gravity roughly 3.0–3.4. The apparent pinkish vein or band may be feldspar, quartz, or alteration material.
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Physical properties
Dark greenish-black, fine- to medium-grained, weakly foliated and elongated, with a dull to subtly vitreous luster; typical hardness is about 5–6 Mohs and specific gravity roughly 3.0–3.4. The apparent pinkish vein or band may be feldspar, quartz, or alteration material.
Formation & geological history
Forms when basaltic or gabbroic protoliths undergo medium- to high-grade regional metamorphism, commonly during mountain building; specimens may range from Precambrian to younger Paleozoic–Mesozoic ages. The rounded shape indicates natural wear or deliberate polishing/tumbling.
Uses & applications
Used as crushed stone, aggregate, riprap, dimension stone, and occasionally decorative material; attractive pieces are collected but rarely used as gemstones.
Geological facts
Amphibolite commonly preserves metamorphic foliation and elongated hornblende crystals, distinguishing it from uniformly dark basalt. Exact identification requires a fresh surface, hardness testing, and ideally thin-section or mineralogical analysis.
Field identification & locations
Look for dense dark rock with aligned amphibole grains, weak layering, and little vesicularity; common in metamorphic belts worldwide. Avoid identifying solely by color because dark gneiss, basalt, serpentinite, and schist can look similar.
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