igneous
Weathered basaltic rock
Basalt, likely altered/vesicular basalt (plagioclase–pyroxene–olivine assemblage)
AI-generated identification · may be incorrectHardness about 5–6 Mohs; dark gray-brown, fine-grained and dull to slightly earthy, with rusty iron-oxide weathering. The specimen appears dense and irregular, with possible small vesicles or mineral-filled cavities; exact composition requires testing.
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Physical properties
Hardness about 5–6 Mohs; dark gray-brown, fine-grained and dull to slightly earthy, with rusty iron-oxide weathering. The specimen appears dense and irregular, with possible small vesicles or mineral-filled cavities; exact composition requires testing.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Its age cannot be determined from the image and may range from recent to billions of years.
Uses & applications
Used as aggregate, roadstone, riprap, building stone, and occasionally as a decorative or lapidary material; this weathered piece has little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Brown coloration commonly results from oxidation of iron-bearing minerals during weathering.
Field identification & locations
Identify it by its fine grain, high density, dark mafic color, and lack of obvious individual crystals; a streak, magnetism, acid, and fresh-surface inspection would help distinguish it from weathered andesite or iron-rich sedimentary rock. Common worldwide, especially volcanic regions and river gravels.
