igneous
Polished basalt cobble
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectFine-grained, dark gray to brown and mottled, with a dull to waxy polish; hardness about 5–6 Mohs, typically massive with no visible cleavage, and specific gravity about 2.8–3.0. The rounded shape indicates water abrasion, while the reflective streak appears polished or naturally smoothed.
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Physical properties
Fine-grained, dark gray to brown and mottled, with a dull to waxy polish; hardness about 5–6 Mohs, typically massive with no visible cleavage, and specific gravity about 2.8–3.0. The rounded shape indicates water abrasion, while the reflective streak appears polished or naturally smoothed.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in oceanic crust and volcanic fields. Individual cobbles may be geologically young or much older, depending on the source lava and river or beach deposit.
Uses & applications
Used as crushed aggregate, road stone, riprap, landscaping rock, and occasionally as a polished decorative or lapidary stone. Common cobbles generally have little commercial value.
Geological facts
Basalt is the most widespread volcanic rock and makes up much of the ocean floor. Its dark color results mainly from iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its dense, fine-grained texture, dark mottling, lack of obvious large crystals, and inability to scratch with a copper coin; it commonly occurs in volcanic terrains, river gravels, and beaches. Exact identification would require a fresh surface or testing because weathered and polished basalt can resemble other dark rocks.
