igneous
Rounded river pebble, likely basaltic or andesitic volcanic rock
Fine-grained mafic to intermediate volcanic rock; likely basalt/andesite, but exact identification requires testing
AI-generated identification · may be incorrectDark gray-green and brown mottling with a dull to weakly satin luster; rounded, smooth surface indicates water abrasion. Typical hardness is about Mohs 5–6, with microscopic interlocking crystals and no obvious cleavage; density commonly 2.7–3.0 g/cm³.
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Physical properties
Dark gray-green and brown mottling with a dull to weakly satin luster; rounded, smooth surface indicates water abrasion. Typical hardness is about Mohs 5–6, with microscopic interlocking crystals and no obvious cleavage; density commonly 2.7–3.0 g/cm³.
Formation & geological history
Probably a weathered volcanic rock transported and rounded in a river, beach, or glacial deposit. Its precise geological age and formation locality cannot be determined from the photograph.
Uses & applications
Usually a decorative or collectible pebble; similar durable volcanic rocks are used as aggregate, landscaping stone, riprap, and occasionally building stone.
Geological facts
The mottled appearance may reflect altered pyroxene, feldspar, iron oxides, or secondary minerals. A streak, magnetism test, and fresh-surface inspection would help distinguish basalt from andesite or other dark rocks.
Field identification & locations
Look for a fine-grained texture, substantial weight, lack of visible layering, and possible weak magnetism; a glassy porcelain-like streak or strong fizz would suggest another material. Common worldwide in volcanic regions and stream gravels.
