igneous
Dark basalt fragment
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, massive, and dull to slightly vitreous; hardness about 5–6 Mohs, with no visible cleavage and a specific gravity near 2.8–3.0. The specimen appears weathered or naturally water-worn, so its exact texture is partly obscured.
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Physical properties
Typically dark gray to black, fine-grained, massive, and dull to slightly vitreous; hardness about 5–6 Mohs, with no visible cleavage and a specific gravity near 2.8–3.0. The specimen appears weathered or naturally water-worn, so its exact texture is partly obscured.
Formation & geological history
Forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and ancient lava flows. Its age cannot be determined from the photograph and may range from recent to billions of years.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, paving stone, and building material; attractive fine-grained pieces are collected but generally have little gem value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Some varieties contain vesicles, olivine crystals, or magnetic iron oxides.
Field identification & locations
Identify it by its black/dark-gray color, heavy feel, fine texture, and resistance to scratching with a steel knife; it commonly occurs in lava-flow regions and stream gravels derived from volcanic terrain. A hand lens and streak/magnet tests can help distinguish it from slag or other dark rocks.
