igneous
Tumbled dark volcanic rock, possibly basalt
Basalt; commonly composed of plagioclase feldspar, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectDark gray to black, fine-grained, and weakly vitreous to dull; hardness about 5–6 Mohs, no obvious cleavage, and typical specific gravity around 2.8–3.0. The glossy surface appears produced by tumbling or polishing, so exact texture is partly obscured.
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Physical properties
Dark gray to black, fine-grained, and weakly vitreous to dull; hardness about 5–6 Mohs, no obvious cleavage, and typical specific gravity around 2.8–3.0. The glossy surface appears produced by tumbling or polishing, so exact texture is partly obscured.
Formation & geological history
Basalt forms when low-viscosity mafic lava cools rapidly at or near Earth’s surface, generally in volcanic fields, mid-ocean ridges, and hotspot environments. Its age cannot be determined from the photograph; basalt ranges from Precambrian to modern.
Uses & applications
Used widely as crushed aggregate, road stone, riprap, paving, and occasionally ornamental stone. Tumbled pieces are inexpensive educational or decorative specimens.
Geological facts
The specimen may alternatively be a dark fine-grained volcanic rock such as andesite or a glassy slag-like material; visual confirmation alone is limited. Basalt is the most abundant volcanic rock on oceanic crust.
Field identification & locations
Common in volcanic regions worldwide, including Hawaii, Iceland, India, and the western United States. Check for tiny interlocking crystals, magnetic response from magnetite, and lack of bubbles; a streak, density, and hardness test can improve identification.
