igneous
Tumbled dark igneous rock, likely basalt
Basalt; predominantly plagioclase feldspar and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectDark gray to brown, fine-grained, massive, and dull to subtly vitreous; Mohs hardness about 5–6, with no obvious cleavage and a typical specific gravity near 2.8–3.0. The rounded surface indicates tumbling or natural water wear.
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Physical properties
Dark gray to brown, fine-grained, massive, and dull to subtly vitreous; Mohs hardness about 5–6, with no obvious cleavage and a typical specific gravity near 2.8–3.0. The rounded surface indicates tumbling or natural water wear.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, mid-ocean ridges, and oceanic islands. Individual specimens may range from recent lava to rocks millions or billions of years old; age cannot be determined visually.
Uses & applications
Used widely as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces are sold as landscaping or lapidary specimens.
Geological facts
The specimen’s dark, fine-grained appearance is consistent with basalt, but the image does not show diagnostic vesicles or crystals, so identification is approximate.
Field identification & locations
Identify basalt by its heavy feel, dark color, fine texture, and lack of visible large crystals; a streak, hardness, magnetism, and thin-section test can improve certainty. Common worldwide, especially volcanic regions.
