igneous
Weathered granitic cobble
Granite to granodiorite; quartz–feldspar–mica aggregate
AI-generated identification · may be incorrectCoarse-grained, speckled tan-brown rock with pale feldspar, translucent gray quartz, and dark mica or amphibole; Mohs hardness ~6–7, vitreous to dull luster, massive texture, no consistent cleavage as a whole, and specific gravity ~2.6–2.8.
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Physical properties
Coarse-grained, speckled tan-brown rock with pale feldspar, translucent gray quartz, and dark mica or amphibole; Mohs hardness ~6–7, vitreous to dull luster, massive texture, no consistent cleavage as a whole, and specific gravity ~2.6–2.8.
Formation & geological history
Formed when silica-rich magma slowly crystallized deep underground, commonly during Precambrian to Phanerozoic plutonic events; surface weathering has rounded and oxidized the cobble.
Uses & applications
Used as crushed aggregate, road stone, building stone, countertops, monuments, and landscaping material; ordinary hand specimens generally have little commercial value.
Geological facts
The brown coloration likely reflects iron-oxide staining and weathering of biotite or amphibole. Individual quartz and feldspar grains should remain visible with a hand lens.
Field identification & locations
Common in glacial deposits, stream gravels, and eroding continental bedrock worldwide. Identify it by its interlocking visible grains, hardness, lack of layering, and inability to scratch quartz with a steel knife; a precise granite versus granodiorite identification requires thin-section or chemical analysis.
