igneous
Weathered granitic or gneissic cobble
Likely altered granite/granodiorite; exact identification requires a fresh surface and testing
AI-generated identification · may be incorrectCoarse, mottled gray-brown rock with weathered feldspar, quartz, and dark mica or amphibole; likely Mohs 6–7, dull to granular luster, massive texture, and weak or absent cleavage as a whole rock. Brown coloration is probably iron-oxide weathering or surface staining.
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Physical properties
Coarse, mottled gray-brown rock with weathered feldspar, quartz, and dark mica or amphibole; likely Mohs 6–7, dull to granular luster, massive texture, and weak or absent cleavage as a whole rock. Brown coloration is probably iron-oxide weathering or surface staining.
Formation & geological history
Probably crystallized slowly from silica-rich magma deep in the crust, potentially hundreds of millions to billions of years ago, then exposed and rounded by erosion or transport. The image does not show enough detail to distinguish granite from granodiorite or foliated gneiss confidently.
Uses & applications
Commonly used as aggregate, riprap, paving stone, and landscaping material; an ordinary specimen has little jewelry value unless it displays attractive veining or unusual mineralization.
Geological facts
Quartz and feldspar grains should be visible on a freshly broken or sawn surface; mica may appear as shiny dark flakes. A magnet test may detect minor magnetite but is not diagnostic.
Field identification & locations
Common in continental crust and glacial or river gravels worldwide; inspect a fresh surface, test hardness against glass, and look for quartz, feldspar, and mica. Estimated value: $0–$10 as a common hand specimen, or $10–$30 if distinctive or polished.
