igneous
Polished dark pebble, likely basalt
Basalt, chiefly plagioclase feldspar and pyroxene, with possible olivine or magnetite
AI-generated identification · may be incorrectFine-grained, dark brown-black, glassy to waxy when wet or polished, with no visible crystals; hardness about 5–6 Mohs, dense, and typically non-cleavable. The rounded shape indicates water abrasion rather than natural crystal form.
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Physical properties
Fine-grained, dark brown-black, glassy to waxy when wet or polished, with no visible crystals; hardness about 5–6 Mohs, dense, and typically non-cleavable. The rounded shape indicates water abrasion rather than natural crystal form.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly from volcanic eruptions or oceanic crust; specimens may range from recent to billions of years old. This pebble was likely transported and smoothed in a stream, lake, or coastal environment.
Uses & applications
Common basalt is used for crushed aggregate, roadstone, railway ballast, building stone, and mineral wool; small polished pieces are mainly decorative or collectible. It has little gemstone value unless unusually patterned or containing attractive olivine.
Geological facts
Dark basalt can resemble gabbro, chert, slag, or weathered iron-rich rocks. A magnet may respond weakly if magnetite is present, while a fresh broken surface should remain uniformly fine-grained.
Field identification & locations
Common in volcanic regions, ocean beaches, river gravels, and flood-basalt provinces worldwide; notable occurrences include Hawaii, Iceland, India’s Deccan Traps, and the Columbia River Basalt Group. Identification is more reliable on a fresh fracture than on a wet polished surface.
