igneous
Likely basalt or fine-grained mafic rock
Basalt; commonly composed of plagioclase feldspar, pyroxene, and sometimes olivine
AI-generated identification · may be incorrectDark gray-green, fine-grained, dull to weakly vitreous surface; typically Mohs hardness 5–6, massive texture, no visible cleavage, and specific gravity about 2.8–3.0. The rounded shape appears naturally weathered or human-tumbled.
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Physical properties
Dark gray-green, fine-grained, dull to weakly vitreous surface; typically Mohs hardness 5–6, massive texture, no visible cleavage, and specific gravity about 2.8–3.0. The rounded shape appears naturally weathered or human-tumbled.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. It may range from Precambrian to modern in age; age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road stone, railroad ballast, riprap, building stone, and occasionally for carvings or decorative specimens; this example has limited jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Look for a uniformly fine-grained dark texture, substantial weight, and lack of obvious layering; a streak, magnetism test, and fresh broken surface would improve identification. Common worldwide, especially volcanic regions and river gravels.
