igneous
Dark volcanic rock, likely basalt
Basalt; predominantly plagioclase feldspar, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectFine-grained, dark gray, dull to subtly silky luster, with rounded water-worn surfaces; hardness about 6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. The specimen appears dense and non-vesicular.
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Physical properties
Fine-grained, dark gray, dull to subtly silky luster, with rounded water-worn surfaces; hardness about 6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. The specimen appears dense and non-vesicular.
Formation & geological history
Basalt forms when low-viscosity mafic lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Its exact age cannot be determined from the image and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, road base, railway ballast, building stone, and occasionally polished for decorative objects. This small rounded piece has little commercial jewelry value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and commonly produces black sand and columnar-jointed lava flows. Weathering may give it brown, reddish, or green alteration colors.
Field identification & locations
Identify it by its uniform dark color, fine texture, substantial weight, and lack of visible quartz or large crystals; a streak, magnetism test, and fresh-break inspection can help distinguish it from slag or other dark rocks. Common worldwide, especially near volcanic regions and beaches or streambeds.
