igneous
Tumbled basalt (likely)
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to subtly glassy luster, typically massive with no visible cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The rounded shape and smooth surface indicate water-worn or tumbled material, while the pale streak may be a weathered vein or attached mineral.
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Physical properties
Dark gray to black, fine-grained, dull to subtly glassy luster, typically massive with no visible cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The rounded shape and smooth surface indicate water-worn or tumbled material, while the pale streak may be a weathered vein or attached mineral.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust. Its age can range from recent eruptions to Precambrian specimens; the image alone cannot establish a precise age.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, paving, and building stone; attractive polished pieces are sold as inexpensive decorative or educational specimens.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its uniform dark color, fine texture, heavy feel, and lack of obvious large crystals; a streak test, magnetism check, and examination of a fresh surface would improve confidence. Common sources include volcanic regions, river gravels, and beaches worldwide.
