igneous
Weathered dark igneous rock fragment
Likely basalt or andesite; exact identification is uncertain from the photograph
AI-generated identification · may be incorrectFine- to medium-grained, dark gray to black, dull to weakly vitreous; likely composed mainly of plagioclase and pyroxene, with possible olivine. Typical hardness is about 6–7 Mohs, with no obvious cleavage; the surrounding material appears to be asphalt containing aggregate.
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Physical properties
Fine- to medium-grained, dark gray to black, dull to weakly vitreous; likely composed mainly of plagioclase and pyroxene, with possible olivine. Typical hardness is about 6–7 Mohs, with no obvious cleavage; the surrounding material appears to be asphalt containing aggregate.
Formation & geological history
Basaltic or andesitic magma cooled rapidly at or near Earth’s surface, forming volcanic rock, generally younger than about 2.5 billion years. The specimen appears weathered and may be road-surface material rather than an intact natural outcrop.
Uses & applications
Common basalt and andesite are used as crushed aggregate, roadstone, riprap, railroad ballast, and building stone; attractive pieces may be collected or polished but are rarely used as gemstones.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly forms oceanic crust and lava flows. The pale grains visible nearby are likely aggregate fragments, not necessarily part of the specimen.
Field identification & locations
Look for a uniformly fine texture, dark color, high density, and absence of visible fossils; basalt may contain tiny vesicles or olivine crystals. Common worldwide, especially volcanic regions; this roadside fragment has negligible market value, typically $0–$5.
