igneous
Tumbled dark pebble, likely basalt
Basalt, predominantly plagioclase feldspar and pyroxene, commonly with olivine or magnetite
AI-generated identification · may be incorrectFine-grained, dark gray to black, dull-to-subvitreous luster, typically hardness 5–6 Mohs and specific gravity about 2.8–3.1; no visible cleavage. The rounded shape indicates natural abrasion or tumbling, and the image alone cannot confirm the identification.
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Physical properties
Fine-grained, dark gray to black, dull-to-subvitreous luster, typically hardness 5–6 Mohs and specific gravity about 2.8–3.1; no visible cleavage. The rounded shape indicates natural abrasion or tumbling, and the image alone cannot confirm the identification.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Most specimens are geologically young relative to continental rocks, ranging from recent eruptions to billions of years old.
Uses & applications
Used as crushed aggregate, roadstone, dimension stone, ballast, and landscaping material; attractive polished pieces are sold as inexpensive decorative stones.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Dark color and fine texture distinguish it from coarse-grained gabbro.
Field identification & locations
Look for a uniformly dark, fine-grained, heavy pebble that does not fizz with dilute acid; a streak, magnetism, or thin-section test can help detect magnetite or confirm composition. Common in volcanic regions, river gravels, beaches, and commercial tumbled-stone mixes.
