igneous
Polished dark pebble, likely basalt
Basalt; mainly plagioclase feldspar, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectFine-grained, dark gray to black, weakly vitreous-to-dull luster, hardness about 5–6 Mohs, and no visible cleavage; the smooth surface indicates tumbling or polishing.
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Physical properties
Fine-grained, dark gray to black, weakly vitreous-to-dull luster, hardness about 5–6 Mohs, and no visible cleavage; the smooth surface indicates tumbling or polishing.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust; specimens may range from recent to billions of years old.
Uses & applications
Used as aggregate, roadstone, building stone, and decorative polished rock; ordinary tumbled pieces have minimal commercial value.
Geological facts
Its high density and dark color reflect abundant iron- and magnesium-rich minerals. Small pale flecks may be plagioclase crystals or weathering products.
Field identification & locations
Identify by its uniformly fine texture, heavy feel, dark color, and lack of distinct grains; it is common in volcanic regions, river gravels, and beaches. Exact identification would require an unpolished surface or mineral testing.
