igneous
Likely tumbled dark volcanic rock
Probably basalt or basaltic andesite; exact identification is uncertain from the image
AI-generated identification · may be incorrectDark brown-black, fine-grained, opaque, and dull to slightly subvitreous; typically hardness 5–6 Mohs, no visible cleavage, and specific gravity about 2.8–3.1. The rounded, smooth shape indicates tumbling or natural abrasion.
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Physical properties
Dark brown-black, fine-grained, opaque, and dull to slightly subvitreous; typically hardness 5–6 Mohs, no visible cleavage, and specific gravity about 2.8–3.1. The rounded, smooth shape indicates tumbling or natural abrasion.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly from volcanic eruptions, oceanic crust, and ancient lava flows. Its age cannot be determined visually and may range from recent to billions of years.
Uses & applications
Used as aggregate, road metal, riprap, ballast, and occasionally as a decorative or lapidary stone. Tumbled pieces have modest collecting value.
Geological facts
Basalt is rich in pyroxene and plagioclase and commonly contains olivine or magnetite, which can make it magnetic. The specimen may also be a dark slag or another volcanic rock if it is unusually glassy or vesicular.
Field identification & locations
Look for a dense, fine texture, very dark color, absence of obvious grains, and possible weak attraction to a magnet; a streak test and fresh fracture help distinguish it from slag. Common worldwide, especially in volcanic regions and oceanic settings.
