metamorphic
Weathered quartz-rich cobble (likely quartzite or siliceous sandstone)
Quartzite, predominantly SiO₂; exact identification requires testing
AI-generated identification · may be incorrectHardness about 7 if quartzite; gray-brown, reddish iron staining, granular texture, and dull to slightly glassy luster. Massive with no obvious cleavage; rounded from stream or beach abrasion. Specific gravity roughly 2.6–2.7.
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Physical properties
Hardness about 7 if quartzite; gray-brown, reddish iron staining, granular texture, and dull to slightly glassy luster. Massive with no obvious cleavage; rounded from stream or beach abrasion. Specific gravity roughly 2.6–2.7.
Formation & geological history
Likely formed from quartz-rich sandstone recrystallized during metamorphism, then eroded and rounded in a sedimentary or glacial environment. Its precise geological age and provenance cannot be determined from the photograph.
Uses & applications
Common quartzite is used as crushed aggregate, road stone, building stone, and occasionally decorative material; this nondescript cobble has little commercial or jewelry value.
Geological facts
Quartzite commonly breaks across quartz grains rather than around them, producing a sugary or splintery fracture. A steel file should not scratch it, while quartz-rich sandstone may shed individual grains more readily.
Field identification & locations
Look for interlocking quartz grains, hardness near 7, and lack of reaction to dilute acid; a streak test is white. Common worldwide, especially in former sandstone terrains and river gravels.
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