igneous
Rounded gray river pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; exact identification requires testing
AI-generated identification · may be incorrectDark gray-brown, dull to subtly satin luster, fine-grained and massive with no visible crystals or cleavage; likely Mohs hardness 5.5–6.5 and specific gravity about 2.7–3.0. The smooth rounded shape reflects water abrasion.
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Physical properties
Dark gray-brown, dull to subtly satin luster, fine-grained and massive with no visible crystals or cleavage; likely Mohs hardness 5.5–6.5 and specific gravity about 2.7–3.0. The smooth rounded shape reflects water abrasion.
Formation & geological history
Probably a weathered fragment of volcanic lava transported and rounded in a river or beach environment; its precise geological age and source cannot be determined from the photograph. Basalt is common in Cenozoic and older volcanic terrains worldwide.
Uses & applications
Used as landscaping stone, garden gravel, decorative aggregate, and occasionally as a tumbling or craft stone; it has little gem value unless unusually patterned or polished.
Geological facts
Basalt commonly contains plagioclase and pyroxene, with possible olivine or magnetite; andesite is generally lighter and more silica-rich. A magnet may respond weakly if magnetite is abundant.
Field identification & locations
Identify it by its uniform fine texture, lack of layering, dark color, and possible magnetic response; a streak, density test, and fresh broken surface would help distinguish basalt from andesite, chert, or metamorphic rock. Common in volcanic regions and streambeds.
