igneous
Unidentified dark gray rock fragment
Likely fine-grained basalt or andesite; exact identification requires testing
AI-generated identification · may be incorrectGray-brown, dull to weakly earthy luster, fine-grained and massive, with no obvious cleavage or visible crystals. Estimated hardness 5–6.5 Mohs and specific gravity about 2.7–3.0.
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Physical properties
Gray-brown, dull to weakly earthy luster, fine-grained and massive, with no obvious cleavage or visible crystals. Estimated hardness 5–6.5 Mohs and specific gravity about 2.7–3.0.
Formation & geological history
Probably volcanic rock formed when lava cooled at or near Earth’s surface, commonly during the Cenozoic or older geological periods; the image alone cannot establish age.
Uses & applications
Basalt and andesite are used as crushed aggregate, road stone, riprap, and building material; attractive pieces may be kept as geological specimens but this fragment has little commercial value.
Geological facts
Its rough, angular shape and dark fine-grained appearance are consistent with a broken volcanic rock, though weathered sedimentary rock is also possible.
Field identification & locations
Look for a dense texture, absence of layering, and tiny interlocking crystals; a streak test, magnet test, hardness test, and fresh broken surface would improve identification. Common worldwide, especially in volcanic regions.
