igneous
Weathered granitic gravel / decomposed granite
Granite-derived granitic saprolite; chiefly quartz, K-feldspar, plagioclase, and mica
AI-generated identification · may be incorrectCoarse, angular pale gray to pink fragments in a sandy matrix, with visible quartz and feldspar grains; vitreous quartz and dull feldspar, typically Mohs 6–7. The loose, granular texture indicates strong weathering rather than a single intact specimen.
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Physical properties
Coarse, angular pale gray to pink fragments in a sandy matrix, with visible quartz and feldspar grains; vitreous quartz and dull feldspar, typically Mohs 6–7. The loose, granular texture indicates strong weathering rather than a single intact specimen.
Formation & geological history
Formed from weathering and disintegration of granite or granodiorite, likely Mesozoic California basement rock; surface weathering produces decomposed granite and residual gravel. The image is consistent with material common around the central California Coast Ranges near 36.9°N, 121.4°W.
Uses & applications
Used as road base, landscaping gravel, paths, aggregate, and building stone when competent; individual fragments generally have negligible collector value.
Geological facts
The rusty or brown tones commonly come from oxidation of iron-bearing biotite and other minerals. Decomposed granite can retain the original rock’s quartz-feldspar composition while losing cohesion.
Field identification & locations
Identify by gritty sandy matrix, angular quartz-rich grains, pale feldspar, and occasional dark mica; a hand lens helps. A firm fresh fragment should not fizz strongly with dilute acid, unlike limestone.
