igneous
Weathered volcanic rock, likely basalt
Basalt; commonly composed of plagioclase, pyroxene, and iron-rich oxides
AI-generated identification · may be incorrectDark olive-brown, fine-grained, and dull to faintly waxy from heavy weathering or surface coating; typically Mohs 5–6, with no visible cleavage and specific gravity about 2.8–3.0. The image shows a massive, rounded specimen with possible vesicles or alteration patches, but mineral identification is limited by the surfa…
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Physical properties
Dark olive-brown, fine-grained, and dull to faintly waxy from heavy weathering or surface coating; typically Mohs 5–6, with no visible cleavage and specific gravity about 2.8–3.0. The image shows a massive, rounded specimen with possible vesicles or alteration patches, but mineral identification is limited by the surface condition.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined from the photograph; the brown-green rind likely reflects chemical weathering and iron oxidation.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, building stone, and occasionally decorative landscaping material; generally low collector value unless unusually fresh, vesicular, or associated with minerals.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the ocean floor. Weathered basalt can resemble altered peridotite, slag, or other dark rocks.
Field identification & locations
Look for a dense fine-grained texture, black or gray fresh surfaces, weak magnetism, and hardness sufficient to scratch glass; a fresh broken surface and streak/magnet tests would improve identification. Common worldwide, especially Hawaii, Iceland, India, and the Pacific Northwest.
