igneous
Dark basalt or fine-grained mafic rock
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine or iron-titanium oxides
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, opaque, and dull to subtly vitreous; Mohs hardness about 5–6, with no obvious cleavage and high density around 2.8–3.0 g/cm³. The specimen appears fractured and weathered, but the image cannot confirm a precise identification.
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Physical properties
Typically dark gray to black, fine-grained, opaque, and dull to subtly vitreous; Mohs hardness about 5–6, with no obvious cleavage and high density around 2.8–3.0 g/cm³. The specimen appears fractured and weathered, but the image cannot confirm a precise identification.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and hotspots; most examples range from recent to hundreds of millions of years old.
Uses & applications
Crushed basalt is widely used as aggregate, road stone, railroad ballast, and construction material; attractive polished varieties are used decoratively, though this fragment has little gem value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the oceanic crust. Magnetic basalt may contain magnetite and can sometimes weakly attract a magnet.
Field identification & locations
Look for a uniformly fine-grained black surface, substantial weight, and absence of visible quartz; a streak test, magnet test, and fresh-break examination help distinguish it from charcoal, slag, or dark sedimentary rocks. Common worldwide, especially volcanic regions and oceanic islands.
