igneous
Likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, clinopyroxene, and often olivine
AI-generated identification · may be incorrectDark gray-green, dull to weakly earthy luster, massive and fine-grained with no visible crystals; hardness about 6, with no obvious cleavage. The rough, dense appearance is consistent with basalt, though weathering obscures diagnostic features.
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Physical properties
Dark gray-green, dull to weakly earthy luster, massive and fine-grained with no visible crystals; hardness about 6, with no obvious cleavage. The rough, dense appearance is consistent with basalt, though weathering obscures diagnostic features.
Formation & geological history
Forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces; individual specimens may be millions to billions of years old.
Uses & applications
Used as crushed aggregate, road base, railway ballast, concrete material, building stone, and occasionally ornamental stone; common hand specimens generally have modest collector value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Green coloration can result from weathering or secondary minerals such as chlorite, epidote, or serpentine.
Field identification & locations
Test for a dense, fine-grained texture, dark streak, and lack of reaction with dilute acid; a steel nail usually will not scratch fresh basalt. Common worldwide, especially volcanic regions and river gravels; exact identification may require a fresh surface or thin section.
