igneous
Tumbled dark stone, likely basalt
Basalt; fine-grained mafic rock composed mainly of plagioclase feldspar and pyroxene, commonly with olivine
AI-generated identification · may be incorrectVery dark gray-green to black, rounded and polished, with a dull to faintly waxy luster; hardness about 5–6 Mohs, fine-grained massive texture, no visible cleavage, and specific gravity roughly 2.8–3.0.
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Physical properties
Very dark gray-green to black, rounded and polished, with a dull to faintly waxy luster; hardness about 5–6 Mohs, fine-grained massive texture, no visible cleavage, and specific gravity roughly 2.8–3.0.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust; its age may range from recent eruptions to billions of years.
Uses & applications
Used as aggregate, roadstone, building stone, ballast, and landscaping material; attractive tumbled pieces are inexpensive decorative or educational specimens.
Geological facts
The image shows a small, uniformly rounded, human-tumbled pebble; exact identification is limited because its texture and minerals are obscured by polishing and low resolution.
Field identification & locations
Commonly found in volcanic regions, river gravels, beaches, and commercial tumbling mixes. Test for a dark streak, lack of fizz in dilute acid, and hardness sufficient to scratch glass; examine a fresh edge for tiny feldspar crystals.
