igneous
Weathered basalt or andesite
Fine- to medium-grained mafic/intermediate volcanic rock; likely plagioclase–pyroxene ± olivine, but species-level identification requires a fresh surface or testing.
AI-generated identification · may be incorrectDark brown-gray, dull to faintly granular, massive and heavily weathered; likely Mohs hardness 5–6, no obvious cleavage, and specific gravity about 2.7–3.1. The rounded triangular form appears natural weathering rather than crystal shape.
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Physical properties
Dark brown-gray, dull to faintly granular, massive and heavily weathered; likely Mohs hardness 5–6, no obvious cleavage, and specific gravity about 2.7–3.1. The rounded triangular form appears natural weathering rather than crystal shape.
Formation & geological history
Formed when lava cooled at or near Earth’s surface, generally during the Cenozoic or older; the image alone cannot establish its geological age or exact volcanic setting.
Uses & applications
Common basalt is used as crushed aggregate, roadstone, riprap, and occasionally building stone; ordinary specimens have little jewelry value.
Geological facts
Basalt is the most widespread volcanic rock on oceanic crust, while andesite commonly forms volcanic arcs. A fresh broken surface, hand lens, streak, magnet, and acid test would improve identification.
Field identification & locations
Look for a fine-grained dark matrix; basalt may contain tiny plagioclase or olivine crystals and can be weakly magnetic. Common worldwide, especially volcanic regions and oceanic terrains.
