igneous
Basalt cobble
Basalt; fine-grained mafic volcanic rock dominated by plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray-brown, dull to subtly glassy, dense, and fine-grained; typically Mohs hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.0. The rounded shape indicates water or abrasion wear; surface appears weathered or possibly coated with soil.
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Physical properties
Dark gray-brown, dull to subtly glassy, dense, and fine-grained; typically Mohs hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.0. The rounded shape indicates water or abrasion wear; surface appears weathered or possibly coated with soil.
Formation & geological history
Forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood basalts. Most examples range from recent to hundreds of millions of years old; age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and occasionally landscaping stone; attractive polished pieces may be used decoratively, but this specimen has little gemstone value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, may later fill with minerals such as zeolite, calcite, or quartz.
Field identification & locations
Identify by its dark color, high density, fine texture, and inability to scratch easily with a steel nail; acid should not fizz. Common in volcanic regions, river gravels, and beaches near basaltic terrain.
