igneous
Polished black stone, likely basalt or dark metamorphic rock
Basalt, predominantly plagioclase feldspar and pyroxene; exact identification requires testing
AI-generated identification · may be incorrectDark gray-black, fine-grained, opaque, and glassy to dull-lustrous when polished; typical hardness is about Mohs 5–6, with no visible cleavage and a dense, compact texture. The elongated subtle markings may indicate flow texture or weathered mineral grains.
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Physical properties
Dark gray-black, fine-grained, opaque, and glassy to dull-lustrous when polished; typical hardness is about Mohs 5–6, with no visible cleavage and a dense, compact texture. The elongated subtle markings may indicate flow texture or weathered mineral grains.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust; specimens may range from recent to Precambrian in age. Its rounded shape and sheen indicate natural water-wear and/or human polishing.
Uses & applications
Used as aggregate, roadstone, building stone, and decorative polished material; this small specimen has mainly educational or ornamental value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly contains iron- and magnesium-rich minerals. A strong magnet response may occur if magnetite is present.
Field identification & locations
Common localities include Hawaii, Iceland, India’s Deccan Traps, and oceanic islands; identify it by its uniformly dark fine grain, high density, and lack of fizz in dilute acid. A streak, magnet, and hardness test can distinguish it from slag or dark minerals.
