metamorphic
Amphibolite / Hornblende Schist
Amphibolite
Hardness: 5-6 on the Mohs scale. Color: Dark green to black with white, cream, or brown mineral streaks (plagioclase feldspar and iron staining). Luster: Sub-vitreous to silky/pearly. Texture: Foliated to nematoblastic (lineated amphibole crystals). Cleavage: Distinct prismatic cleavage in amphibole grains.
- Hardness
- 5-6 on the Mohs scale
- Luster
- Sub-vitreous to silky/pearly
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Physical properties
Hardness: 5-6 on the Mohs scale. Color: Dark green to black with white, cream, or brown mineral streaks (plagioclase feldspar and iron staining). Luster: Sub-vitreous to silky/pearly. Texture: Foliated to nematoblastic (lineated amphibole crystals). Cleavage: Distinct prismatic cleavage in amphibole grains.
Formation & geological history
Formed through regional metamorphism of mafic igneous rocks (such as basalt or gabbro) or clay-rich marls under high temperature and moderate-to-high pressure conditions.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and occasionally as decorative dimension stone in architecture. Also collected as educational rock specimens.
Geological facts
Amphibolites make up a significant portion of ancient continental crust and metamorphic terranes, often revealing the tectonic history of mountain-building events (orogenies).
Field identification & locations
Identified in the field by its dark color, lineated or foliated alignment of dark needle-like amphibole (hornblende) crystals, and alternating light feldspar streaks. Commonly found in metamorphic mountain belts like the Appalachians and Alps. Value: $2-$10 for hand-sized field specimens.
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