igneous
Weathered basalt or volcanic breccia
Fine-grained mafic volcanic rock, likely basalt (plagioclase–pyroxene ± olivine), possibly brecciated
AI-generated identification · may be incorrectDark gray to purplish, dull to faintly waxy luster, massive and fine-grained with weathering pits and rusty alteration; hardness about 5–6, no obvious cleavage, specific gravity roughly 2.8–3.1. The rounded shape suggests water abrasion or prolonged weathering.
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Physical properties
Dark gray to purplish, dull to faintly waxy luster, massive and fine-grained with weathering pits and rusty alteration; hardness about 5–6, no obvious cleavage, specific gravity roughly 2.8–3.1. The rounded shape suggests water abrasion or prolonged weathering.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, likely during a Cenozoic or older volcanic episode; surface oxidation and fluid alteration produced the purple-brown and yellowish patches. Exact age cannot be determined from the photograph.
Uses & applications
Common basalt is used as crushed aggregate, road stone, riprap, and occasionally building stone; this small weathered specimen has mainly educational or modest collecting value.
Geological facts
Basalt is the most widespread volcanic rock and forms much of oceanic crust. A magnet may show weak attraction if the rock contains abundant magnetite.
Field identification & locations
Look for a heavy, fine-grained dark rock with tiny interlocking crystals, lacking visible quartz and usually lacking strong layering; a fresh broken surface and streak/magnet tests help distinguish it from altered sedimentary rocks. Common worldwide, especially volcanic islands and oceanic terrains.
