igneous
Tumbled dark pebble, likely basalt
Basalt, predominantly plagioclase feldspar and pyroxene, with possible magnetite
AI-generated identification · may be incorrectFine-grained, opaque, dark gray to black, with a matte-to-subtle satin luster and smoothly water-worn or tumbled surfaces. Hardness is approximately 5–6 Mohs; cleavage is not visible, and specific gravity is typically 2.8–3.0.
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Physical properties
Fine-grained, opaque, dark gray to black, with a matte-to-subtle satin luster and smoothly water-worn or tumbled surfaces. Hardness is approximately 5–6 Mohs; cleavage is not visible, and specific gravity is typically 2.8–3.0.
Formation & geological history
Likely formed as an extrusive volcanic rock when basaltic lava cooled rapidly, then became rounded by stream, beach, or glacial abrasion; its exact geological age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, paving stone, landscaping rock, and occasionally as a decorative or lapidary pebble. This specimen has little jewelry value unless it proves to be a rarer dark gemstone or unusually fine material.
Geological facts
The smooth rounded shape indicates transport or deliberate tumbling rather than a fresh volcanic surface. Similar-looking pebbles can include fine-grained andesite, chert, slag, or dark metamorphic rocks.
Field identification & locations
Look for a uniformly fine texture, no visible glassy bubbles, and a weak response to a magnet if magnetite-rich; a streak test, hardness test, and specific-gravity measurement would improve identification. Common in volcanic regions, river gravels, and beaches.
