igneous
Dark mafic rock, likely basalt
Basalt; chiefly plagioclase feldspar and pyroxene, commonly with minor olivine and magnetite
AI-generated identification · may be incorrectFine-grained, dark gray to black, dull to weakly subvitreous luster, and typically massive with no obvious cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The specimen appears weathered and may contain small vesicles or alteration patches.
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Physical properties
Fine-grained, dark gray to black, dull to weakly subvitreous luster, and typically massive with no obvious cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The specimen appears weathered and may contain small vesicles or alteration patches.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows, volcanic islands, and oceanic crust. It may range from Precambrian to modern in age; the image alone cannot establish age or locality.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, railroad ballast, and dimension stone; attractive dense varieties are polished for decorative surfaces. Ordinary hand specimens have modest collector value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the oceanic crust. Its iron- and magnesium-rich minerals commonly weather to reddish-brown surfaces or clay minerals.
Field identification & locations
Identify it by its uniformly dark, fine-grained texture, heavy feel, and lack of visible large crystals; a streak, magnetism test, and fresh broken surface help distinguish it from slag or shale. Common worldwide, especially volcanic regions and oceanic settings.
