igneous
Dark gray weathered pebble, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectFine-grained, dark gray to nearly black, generally dull to weakly subvitreous, with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rounded shape and surface texture indicate stream or beach abrasion and weathering.
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Physical properties
Fine-grained, dark gray to nearly black, generally dull to weakly subvitreous, with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rounded shape and surface texture indicate stream or beach abrasion and weathering.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, flood-basalt provinces, and mid-ocean ridges. Its exact age cannot be determined from the image; basalt ranges from modern lava to rocks billions of years old.
Uses & applications
Crushed basalt is used for aggregate, road base, riprap, and mineral wool; intact pieces are mainly collected or used decoratively rather than as gemstones.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly forms rounded cobbles in volcanic or mixed gravel environments.
Field identification & locations
Identify it by its uniform dark fine grain, high density, and lack of obvious layering or large crystals; a streak test, magnet response, and hand lens can help distinguish it from dark sandstone, andesite, or slag. Common sources include volcanic terrains and stream/beach gravels.
