igneous
Black pebble, likely basalt or magnetite-rich stone
Probably fine-grained basalt (plagioclase–pyroxene), possibly magnetite-bearing; exact identification requires testing
AI-generated identification · may be incorrectDark brown-black, smooth and rounded, with a dull to subtly waxy luster and no visible crystals; likely hardness ~5–6 Mohs and specific gravity ~2.8–3.3. The image cannot confirm cleavage, grain size, or magnetism.
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Physical properties
Dark brown-black, smooth and rounded, with a dull to subtly waxy luster and no visible crystals; likely hardness ~5–6 Mohs and specific gravity ~2.8–3.3. The image cannot confirm cleavage, grain size, or magnetism.
Formation & geological history
If basalt, it formed when mafic lava cooled rapidly at or near Earth’s surface, commonly in Cenozoic to older volcanic terrains; subsequent stream or beach transport rounded it. A magnetite-rich pebble could instead be an iron-oxide mineral or slag, requiring testing.
Uses & applications
Ordinary basalt pebbles are used as aggregate, landscaping stone, and decorative tumbled material; magnetite specimens are collected and used industrially as iron ore. This small piece has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust. A strong attraction to a magnet would favor magnetite-rich basalt or magnetite; a streak test and specific-gravity measurement would help distinguish them.
Field identification & locations
Common in volcanic regions, river gravels, and beaches worldwide; examine for tiny crystals, vesicles, metallic luster, and magnetic response. Avoid calling it slag unless it shows glassy texture, bubbles, or unnatural flow features.
