igneous
Weathered dark igneous rock, likely basalt
Basalt; chiefly plagioclase feldspar and pyroxene, possibly olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, fine-grained, irregularly fractured, with a dull to slightly glassy luster and apparent weathering rind. Mohs hardness about 5–6; typically massive, with no visible cleavage and specific gravity around 2.8–3.0.
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Physical properties
Dark gray-brown, fine-grained, irregularly fractured, with a dull to slightly glassy luster and apparent weathering rind. Mohs hardness about 5–6; typically massive, with no visible cleavage and specific gravity around 2.8–3.0.
Formation & geological history
Basalt forms when low-silica basaltic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and ancient lava flows. Its exact geological age cannot be determined from the photograph.
Uses & applications
Used widely as crushed aggregate, roadstone, railroad ballast, riprap, and building stone; attractive polished pieces may be collected but this specimen has little gemstone value.
Geological facts
The pale patch may be weathered feldspar, a secondary mineral, or a small cavity filling; image-only identification is not definitive. Basalt commonly contains tiny crystals that require magnification to resolve.
Field identification & locations
Common worldwide, especially in Iceland, Hawaii, India’s Deccan Traps, the Columbia River Basalt Group, and ocean floors. Field tests include a dense dark color, fine grain, lack of fizz with dilute acid, and possible attraction to a strong magnet if iron-rich.
