igneous
Polished dark stone, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black, with a glassy-to-dull polished luster; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity 2.8–3.0. The rounded shape indicates tumbling or water wear.
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Physical properties
Fine-grained, dark gray to black, with a glassy-to-dull polished luster; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity 2.8–3.0. The rounded shape indicates tumbling or water wear.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic fields and oceanic crust. It may range from recent lava to ancient specimens billions of years old.
Uses & applications
Used widely as crushed aggregate, roadstone, building stone, and occasionally as polished decorative or worry stones; it has little jewelry value.
Geological facts
Its dark color comes from iron- and magnesium-rich minerals. A magnet may show weak attraction if the basalt contains magnetite.
Field identification & locations
Identify it by its uniformly fine texture, high density, dark color, and lack of obvious crystals; it can resemble polished slag, jasper, or dark glass, so a streak and hardness test can help. Common worldwide, especially near volcanic regions.
