metamorphic

Rounded quartzite cobble (probable)

Quartzite; predominantly microcrystalline quartz (SiO₂)

AI-generated identification · may be incorrect

Hardness about 7 Mohs; tan-gray, opaque, weathered surface, dull to slightly vitreous luster, and massive granular texture with no obvious cleavage. The rounded edges suggest natural stream or glacial abrasion, though the image cannot exclude chert or a dense sandstone.

Identified September 11, 2026More metamorphic →

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Physical properties

Formation & geological history

Quartzite forms when quartz-rich sandstone is recrystallized during metamorphism, commonly during mountain-building; its age depends on the original sandstone and metamorphic event. It commonly occurs in resistant ridges, river gravels, glacial deposits, and mountain terrains.

Uses & applications

Used as crushed aggregate, road stone, building stone, and occasionally as a lapidary or decorative material; this ordinary cobble has minimal commercial value.

Geological facts

Quartzite commonly fractures across individual grains rather than crumbling around them. A steel file or knife should not readily scratch it, while quartzite can scratch ordinary glass.

Field identification & locations

Look for a very hard, sugary or fused quartz texture and abundant quartz grains; acid produces no reaction unless carbonate cement is present. Common sources include the Appalachian Mountains, Rocky Mountains, Alps, and many glacial or river deposits.