igneous
Dark gray crystalline rock, possibly basalt
Basalt (tentative; plagioclase–pyroxene–iron oxide assemblage)
AI-generated identification · may be incorrectFine-grained, dark gray to black, dull to slightly subvitreous luster; likely hardness ~5–6 Mohs, with no obvious cleavage. The visible pale patches may be weathered feldspar or a different mineral inclusion.
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Physical properties
Fine-grained, dark gray to black, dull to slightly subvitreous luster; likely hardness ~5–6 Mohs, with no obvious cleavage. The visible pale patches may be weathered feldspar or a different mineral inclusion.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly during volcanic eruptions; individual pieces may range from modern to Precambrian in age.
Uses & applications
Used as crushed aggregate, roadstone, building stone, and occasionally as ornamental material; ordinary specimens have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the oceanic crust, including mid-ocean ridges and many flood-basalt provinces.
Field identification & locations
Commonly found as dense, dark, angular volcanic fragments with tiny crystals; a fresh black streak, lack of fizz in dilute acid, and visible interlocking grains support basalt. The small image prevents confident separation from gabbro, hornfels, or a dark mineral-rich metamorphic rock.
