metamorphic

Rounded tan cobble, likely quartzite or sandstone

Quartzite (mostly quartz, SiO₂); possibly quartz-rich sandstone

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs if quartzite; tan to cream, fine-grained, dull to weakly glassy luster, massive texture, and no obvious cleavage. Its smooth rounded shape reflects water abrasion rather than crystal form.

Identified September 2, 2026More metamorphic →

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Physical properties

Formation & geological history

Quartzite forms when quartz-rich sandstone is recrystallized during regional metamorphism, commonly hundreds of millions to billions of years ago; a sandstone identity would indicate sedimentary cementation instead. It was likely transported and rounded in a stream or glacial deposit.

Uses & applications

Ordinary cobbles have little commercial value, though quartzite is used as aggregate, riprap, paving, and decorative stone; attractive pieces may be used in landscaping or lapidary work.

Geological facts

Quartzite commonly resists weathering better than surrounding rocks, producing prominent ridges and durable stream cobbles. A steel file or knife should leave little or no mark on true quartzite.

Field identification & locations

Common in rivers, gravel bars, glacial till, and areas underlain by metamorphic terranes; inspect for interlocking quartz grains and a sugary fracture. A weak acid reaction would suggest carbonate rock instead, while a scratch test helps distinguish it from softer sandstone.