igneous
Polished black pebble, likely basalt
Basalt; predominantly plagioclase feldspar, pyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectFine-grained, dark gray to black, smooth polished surface with a dull to vitreous luster; Mohs hardness about 5–6, no visible cleavage, and specific gravity roughly 2.8–3.0. The rounded shape indicates tumbling or natural abrasion.
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Physical properties
Fine-grained, dark gray to black, smooth polished surface with a dull to vitreous luster; Mohs hardness about 5–6, no visible cleavage, and specific gravity roughly 2.8–3.0. The rounded shape indicates tumbling or natural abrasion.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic regions and oceanic crust. Individual specimens may range from recent volcanic material to ancient rocks billions of years old.
Uses & applications
Used as aggregate, paving stone, railroad ballast, and decorative stone; polished pieces are inexpensive craft or worry stones rather than typical gemstones.
Geological facts
The image lacks diagnostic detail, so black chert, glassy volcanic rock, or another dark stone is also possible. A strong response to a magnet may indicate magnetite-rich basalt.
Field identification & locations
Common in Hawaii, Iceland, the Pacific Northwest, India, and oceanic volcanic terrains. Identification is improved by checking for tiny crystals, streak (usually gray), density, and whether it scratches glass; avoid treating surface polish as a natural luster.
