igneous
Weathered red-brown volcanic rock, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; commonly plagioclase ± pyroxene, iron oxides, and alteration minerals
AI-generated identification · may be incorrectDark reddish-brown, massive, fine-grained, and dull to slightly earthy; approximate hardness 5–6 Mohs, with no obvious cleavage. Pale green-gray patches are likely lichen or surface alteration, so exact identification requires a fresh interior or testing.
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Physical properties
Dark reddish-brown, massive, fine-grained, and dull to slightly earthy; approximate hardness 5–6 Mohs, with no obvious cleavage. Pale green-gray patches are likely lichen or surface alteration, so exact identification requires a fresh interior or testing.
Formation & geological history
Formed when lava cooled at or near Earth’s surface, likely during the Cenozoic or older; the red color commonly results from oxidation of iron-bearing minerals during weathering. Its rounded shape indicates transport or prolonged erosion.
Uses & applications
Used as crushed aggregate, road material, riprap, and occasionally decorative landscaping stone; generally low jewelry or collector value.
Geological facts
Basalt and andesite can look nearly identical in weathered hand specimens; silica content and mineral proportions distinguish them. A streak, density, thin section, or chemical analysis would improve identification.
Field identification & locations
Common in volcanic regions worldwide, including Hawaii, Iceland, the Cascades, Japan, and continental volcanic belts. Look for a dense, fine-grained interior, faint vesicles, and magnetism; avoid identifying from the weathered exterior alone.
