metamorphic

Weathered quartzite cobble

Quartzite; predominantly microcrystalline quartz (SiO₂)

AI-generated identification · may be incorrect

Typically very hard (Mohs 7), gray to tan with rusty iron-oxide staining, dull to slightly glassy luster, and no visible cleavage; grains are tightly fused and may show sugary fracture surfaces. The rounded shape indicates abrasion as a transported cobble.

Identified September 4, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when quartz-rich sandstone was metamorphosed under heat and pressure, commonly during mountain-building; its exact geological age cannot be determined from the image. Later erosion transported and rounded it in a stream, beach, or glacial deposit.

Uses & applications

Used as crushed aggregate, road stone, riprap, and durable building or paving stone; attractive pieces may be used decoratively or collected, but this example appears ordinary.

Geological facts

Quartzite commonly resists weathering better than surrounding rocks, producing resistant ridges and cobbles. Iron oxidation creates the yellow-brown surface patches visible here.

Field identification & locations

Identify it by testing whether it scratches glass and by its fused quartz grains; unlike sandstone, it does not readily crumble along grain boundaries. Common in metamorphic belts and downstream gravel deposits worldwide.