igneous
Rounded dark cobble, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and often olivine
AI-generated identification · may be incorrectDark gray-green, fine-grained, dull to weakly subvitreous, with no visible crystals; typically hardness 5–6 Mohs, massive structure, and specific gravity about 2.8–3.0. Its smooth shape is from water or abrasion rather than crystal form.
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Physical properties
Dark gray-green, fine-grained, dull to weakly subvitreous, with no visible crystals; typically hardness 5–6 Mohs, massive structure, and specific gravity about 2.8–3.0. Its smooth shape is from water or abrasion rather than crystal form.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Individual specimens may be geologically ancient or geologically recent; the image alone cannot establish age.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces are used decoratively but have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly produces dark, iron- and magnesium-rich soils through weathering.
Field identification & locations
Identify it by its dark color, fine texture, high density, and lack of obvious layering or large crystals; a streak, magnetism test, and fresh broken surface help distinguish it from shale, slag, or weathered concrete. Common in volcanic regions and river or beach gravels.
