igneous
Polished basalt pebble
Basalt, a fine-grained mafic volcanic rock composed chiefly of plagioclase feldspar and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectHardness about 6–7 Mohs; dark brown to black with a subdued vitreous-to-waxy luster, very fine-grained texture, and no visible cleavage. Its rounded, glossy surface indicates natural water abrasion and polishing or tumbling; reddish-brown areas may reflect iron oxidation.
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Physical properties
Hardness about 6–7 Mohs; dark brown to black with a subdued vitreous-to-waxy luster, very fine-grained texture, and no visible cleavage. Its rounded, glossy surface indicates natural water abrasion and polishing or tumbling; reddish-brown areas may reflect iron oxidation.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust and volcanic fields; specimens may range from recent lava to Precambrian material. The smooth shape formed by transport in a river, beach, or glacial deposit.
Uses & applications
Used as aggregate, road stone, riprap, paving, and decorative landscaping material; polished pieces are inexpensive curios or cabochon-like ornaments.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the ocean floor. It is denser and darker than granite because it contains abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its high density, dark fine-grained appearance, lack of large crystals, and inability to scratch it with a steel knife easily; a streak test is generally gray to black. Common in volcanic regions, river gravels, and beaches worldwide.
