igneous
Weathered basalt (probable)
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark brown to gray, dull-to-subvitreous, massive and fine-grained; typical hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.1. The brown coloration likely reflects weathering and iron oxidation; the photograph is not sufficient to confirm mineral grains or vesicles.
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Physical properties
Dark brown to gray, dull-to-subvitreous, massive and fine-grained; typical hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.1. The brown coloration likely reflects weathering and iron oxidation; the photograph is not sufficient to confirm mineral grains or vesicles.
Formation & geological history
Forms when basaltic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows and oceanic crust. Most examples range from recent volcanic rocks to Precambrian specimens, but age cannot be determined visually.
Uses & applications
Used as crushed aggregate, roadstone, riprap, railroad ballast, and occasionally dimension stone; generally has little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Look for a dense, dark, fine-grained texture and test hardness against glass; a streak, fresh broken surface, magnetism, and acid reaction can help distinguish it from weathered slag or other rocks. Common worldwide, especially volcanic regions and river gravels.
