igneous
Tumbled black stone, likely basalt
Basalt; fine-grained plagioclase–pyroxene rock, possibly with magnetite
AI-generated identification · may be incorrectDark gray to black, smoothly tumbled, with a dull to faintly waxy luster and no visible crystals; typical basalt hardness is about 6 Mohs, with high specific gravity (~2.8–3.0) and negligible cleavage.
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Physical properties
Dark gray to black, smoothly tumbled, with a dull to faintly waxy luster and no visible crystals; typical basalt hardness is about 6 Mohs, with high specific gravity (~2.8–3.0) and negligible cleavage.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic regions and oceanic crust. Its exact geological age cannot be determined from the photograph.
Uses & applications
Used as aggregate, road stone, riprap, and occasionally as a decorative or lapidary material; tumbled pieces are inexpensive collectibles.
Geological facts
The specimen’s uniform dark color and rounded, polished form are consistent with a commercially tumbled basalt or another dark volcanic rock; identification is not definitive from appearance alone.
Field identification & locations
Test for a glassy streak: basalt generally leaves little or no streak on unglazed ceramic and is not strongly magnetic, though magnetite-bearing pieces may attract a magnet. Common sources include volcanic areas worldwide, especially Hawaii, Iceland, India, and the Pacific Northwest.
