igneous
Likely basalt or gabbro
Mafic igneous rock, chiefly plagioclase feldspar and pyroxene; possible olivine
AI-generated identification · may be incorrectDark gray-black, fine- to medium-grained, dull to weakly subvitreous luster, and typically massive with no obvious cleavage as a whole. Hardness is about 5–6, with specific gravity roughly 2.8–3.2; the pale weathered patch may be feldspar or alteration.
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Physical properties
Dark gray-black, fine- to medium-grained, dull to weakly subvitreous luster, and typically massive with no obvious cleavage as a whole. Hardness is about 5–6, with specific gravity roughly 2.8–3.2; the pale weathered patch may be feldspar or alteration.
Formation & geological history
Basalt forms from rapidly cooled mafic lava, whereas gabbro forms from slowly cooled magma underground; an image alone cannot reliably distinguish them. Such rocks range from modern volcanic material to Precambrian specimens, depending on locality.
Uses & applications
Used as crushed aggregate, road base, riprap, and dimension stone; attractive polished pieces may be sold as landscaping or display specimens rather than jewelry.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of oceanic crust. Gabbro has the same broad chemistry but visibly larger interlocking crystals.
Field identification & locations
Look for abundant dark minerals with scattered gray-white plagioclase; a hand lens can separate fine basalt from coarse gabbro. Common sources include volcanic regions, oceanic crust, and mafic intrusions worldwide.
