igneous
Likely basalt fragments
Basalt, chiefly plagioclase feldspar and pyroxene, with possible olivine or iron oxides
AI-generated identification · may be incorrectDark gray to black, fine-grained and mostly dull, with uneven to conchoidal fractures; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. Some pieces show vesicles or weathered metallic-looking surfaces.
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Physical properties
Dark gray to black, fine-grained and mostly dull, with uneven to conchoidal fractures; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. Some pieces show vesicles or weathered metallic-looking surfaces.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces; individual specimens may range from recent to Precambrian. The fragments appear naturally broken, though exact identification is limited by the image.
Uses & applications
Used extensively as crushed aggregate, roadstone, railway ballast, riprap, paving, and basalt fiber; attractive polished pieces may be used decoratively but are rarely jewelry gemstones.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Magnetic basalt may contain magnetite and can weakly attract a magnet.
Field identification & locations
Identify by its black fine-grained texture, high density, lack of visible large crystals, and possible magnetism; common localities include Hawaii, Iceland, India’s Deccan Traps, and oceanic volcanic regions. Value is generally $1–$10 for ordinary fragments, or about $10–$50 for unusual vesicular or collector specimens.
