igneous
Polished black volcanic rock, likely basalt or basaltic scoria
Basalt (fine-grained mafic silicate rock; commonly plagioclase, pyroxene, and iron oxides)
AI-generated identification · may be incorrectDark gray-black, highly polished, with scattered pale mineral grains and possible vesicles or weathering pits; vitreous to greasy surface after polishing. Basalt typically has Mohs hardness 5–6, no consistent cleavage, fine-grained interlocking crystals, and specific gravity about 2.8–3.1.
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Physical properties
Dark gray-black, highly polished, with scattered pale mineral grains and possible vesicles or weathering pits; vitreous to greasy surface after polishing. Basalt typically has Mohs hardness 5–6, no consistent cleavage, fine-grained interlocking crystals, and specific gravity about 2.8–3.1.
Formation & geological history
Forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows; most examples are Cenozoic to recent, though basalt occurs throughout geological time. The pictured piece appears human-polished.
Uses & applications
Used as aggregate, road stone, riprap, building stone, and decorative/tumbled material; attractive pieces are sold as worry stones or curios. Common specimens generally have modest collector value.
Geological facts
The light specks may be plagioclase crystals, alteration minerals, or polishing residue; the image alone cannot distinguish basalt from related dark volcanic rocks such as andesite or scoria.
Field identification & locations
Look for a dense dark color, fine grain, dull natural fracture, and absence of obvious quartz; a streak is usually gray and weak. Common localities include Hawaii, Iceland, India, the western United States, and oceanic volcanic regions.
