igneous
Likely basaltic rock fragment
Fine-grained mafic igneous rock, probably basalt (plagioclase–pyroxene ± olivine)
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to subvitreous luster, massive texture, and no visible cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. The irregular angular form may be a broken or weathered fragment.
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Physical properties
Dark gray to black, fine-grained, dull to subvitreous luster, massive texture, and no visible cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. The irregular angular form may be a broken or weathered fragment.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic fields and oceanic crust. Its exact age cannot be determined from the photograph; basalt ranges from ancient to recent.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and occasionally dimension stone; attractive polished pieces may be collected but this fragment has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. A small metallic-looking fleck could be pyrite, magnetite, or mica, but it is not diagnostic visually.
Field identification & locations
Look for a uniformly fine texture, dark color, heavy feel, and possible tiny vesicles; test with a steel nail and inspect a fresh surface. Common worldwide, especially volcanic regions and river gravels.
