metamorphic
Weathered quartz-rich cobble (likely quartzite)
Quartzite; predominantly microcrystalline quartz (SiO₂)
AI-generated identification · may be incorrectHardness about 7 Mohs; pale gray, massive, dull to slightly vitreous luster, with rusty iron-oxide staining and faint quartz-rich veining. Quartzite has interlocking quartz grains and no obvious cleavage; specific gravity is about 2.6–2.7.
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Physical properties
Hardness about 7 Mohs; pale gray, massive, dull to slightly vitreous luster, with rusty iron-oxide staining and faint quartz-rich veining. Quartzite has interlocking quartz grains and no obvious cleavage; specific gravity is about 2.6–2.7.
Formation & geological history
Formed when quartz-rich sandstone was deeply buried and metamorphosed, recrystallizing its grains; the original sandstone may be Precambrian or younger, but the specimen’s age cannot be determined from the image. It was likely transported and rounded by stream or surface erosion.
Uses & applications
Used as crushed stone, road aggregate, riprap, building stone, and occasionally decorative or lapidary material. This ordinary weathered cobble has little commercial value.
Geological facts
Quartzite commonly resists weathering more strongly than surrounding rocks, forming ridges, resistant boulders, and stream cobbles. Iron staining develops where water oxidizes minor pyrite or iron-bearing minerals.
Field identification & locations
Identify it by its very high hardness, quartz-like texture, lack of visible sedimentary layering, and inability to scratch it with a steel knife; it should scratch glass. Common in quartzite-bearing terrains and gravel deposits worldwide.
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