igneous
Weathered river cobble, likely basaltic or andesitic volcanic rock
Fine-grained mafic to intermediate igneous rock; exact identification requires testing
AI-generated identification · may be incorrectDark gray-brown, dull to weakly earthy luster, rounded by abrasion, with fine grain and possible altered fracture veins. Typical hardness is about 5–6 Mohs; cleavage is absent and specific gravity commonly 2.7–3.0.
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Physical properties
Dark gray-brown, dull to weakly earthy luster, rounded by abrasion, with fine grain and possible altered fracture veins. Typical hardness is about 5–6 Mohs; cleavage is absent and specific gravity commonly 2.7–3.0.
Formation & geological history
Likely formed when volcanic lava cooled rapidly at or near Earth’s surface, then was weathered and rounded in a stream or shoreline environment. Its exact geological age and formation locality cannot be determined from the image.
Uses & applications
Primarily a natural specimen, paperweight, or teaching sample; comparable basalt and andesite are used as aggregate, roadstone, riprap, and building stone.
Geological facts
The brown coloration may result from iron oxidation and surface weathering, while the dark seams could be fractures filled or stained by secondary minerals.
Field identification & locations
Look for a very fine crystalline texture, lack of visible quartz, and a dark streak or gray fresh interior; a magnet may show weak attraction if magnetite is present. Common sources include volcanic terrains and river gravels worldwide.
