igneous
Possible basalt or industrial iron-rich slag
Basalt (plagioclase–pyroxene rock); identification uncertain from image
AI-generated identification · may be incorrectDark black, fine-grained to glassy, irregularly rounded, with a dull to subtly glassy luster. Basalt typically has Mohs hardness 5–6, no obvious cleavage, and specific gravity about 2.8–3.0; vesicles may occur, while slag can be shinier and more porous.
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Physical properties
Dark black, fine-grained to glassy, irregularly rounded, with a dull to subtly glassy luster. Basalt typically has Mohs hardness 5–6, no obvious cleavage, and specific gravity about 2.8–3.0; vesicles may occur, while slag can be shinier and more porous.
Formation & geological history
Basalt forms when mafic lava rapidly cools at Earth’s surface, commonly in volcanic fields and oceanic crust; ages range from recent to billions of years. The specimen’s smooth, lumpy appearance also permits human-made slag, which is not a natural rock.
Uses & applications
Basalt is used as crushed aggregate, road stone, paving, and occasionally as a collectible specimen. Slag has mainly industrial or educational interest and minimal gem value.
Geological facts
Field distinction is difficult because black basalt and iron-rich furnace slag can look similar, especially when weathered or tumbled.
Field identification & locations
Check for vesicles, tiny plagioclase crystals, and uniform fine texture; test magnetism and streak, and inspect for bubbly glass or metal droplets suggesting slag. Common natural sources include Hawaii, Iceland, India, and oceanic volcanic regions.
