igneous
Dark gray basalt or fine-grained mafic rock
Basalt; predominantly plagioclase feldspar and pyroxene, with possible olivine and magnetite
AI-generated identification · may be incorrectHardness about 6 Mohs; dark gray, fine-grained, dull to faintly vitreous luster, massive texture, and no visible cleavage. The rounded shape suggests abrasion or weathering; surface soil obscures diagnostic features.
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Physical properties
Hardness about 6 Mohs; dark gray, fine-grained, dull to faintly vitreous luster, massive texture, and no visible cleavage. The rounded shape suggests abrasion or weathering; surface soil obscures diagnostic features.
Formation & geological history
Basalt forms when low-silica mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Individual specimens may range from ancient Precambrian rocks to recent eruptions.
Uses & applications
Used as crushed aggregate, road stone, railway ballast, riprap, and occasionally dimension stone; attractive polished pieces are collected but are rarely used as gemstones.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly contains tiny crystals invisible without magnification. A magnet may show weak attraction if magnetite is present.
Field identification & locations
Identify it by its uniformly dark, dense, fine-grained appearance, high weight, and lack of layering; a fresh broken surface is more diagnostic than this soil-covered face. Common worldwide, especially near volcanic regions and river or glacial gravels.
