metamorphic
Weathered iron-stained quartzite or sandstone cobble
Quartzite (quartz, SiO₂), possibly ferruginous sandstone
AI-generated identification · may be incorrectHardness about 6.5–7 for quartzite; dull to slightly granular luster, gray-brown surface with yellow-orange limonite/goethite staining, and no obvious cleavage. The rounded shape and rough granular texture indicate abrasion and weathering; exact identification requires a fresh break or acid test.
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Physical properties
Hardness about 6.5–7 for quartzite; dull to slightly granular luster, gray-brown surface with yellow-orange limonite/goethite staining, and no obvious cleavage. The rounded shape and rough granular texture indicate abrasion and weathering; exact identification requires a fresh break or acid test.
Formation & geological history
Likely a silica-rich sedimentary rock metamorphosed into quartzite, with iron minerals oxidized during surface weathering; alternatively, it may be strongly cemented sandstone. Its age cannot be determined from the photograph and depends on the local bedrock and sediment source.
Uses & applications
Quartzite is used as crushed stone, aggregate, decorative building stone, and occasionally as a durable ornamental material; this ordinary cobble has little commercial value.
Geological facts
Iron oxidation produces the yellow, ochre, and brown coatings. Quartz-rich rocks commonly resist weathering, leaving rounded, durable cobbles in streambeds, glacial deposits, and eroded uplands.
Field identification & locations
Test with a steel nail or knife: quartzite should resist scratching; a fresh surface should show abundant interlocking quartz grains and usually scratch glass. Common in stream gravels and metamorphic terrains; vinegar should not fizz unless carbonate cement is present.
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