metamorphic

Banded quartzite cobble

Quartzite, predominantly microcrystalline quartz (SiO₂)

AI-generated identification · may be incorrect

Hardness about 7 Mohs; gray-green to tan with reddish iron-oxide bands, dull to slightly vitreous luster, and no obvious cleavage. Dense, fine-grained, with curved weathering surfaces and possible faint foliation or sedimentary layering.

Identified September 6, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when quartz-rich sandstone was metamorphosed and recrystallized under heat and pressure, commonly during mountain building; its exact geological age cannot be determined from the image. Rounded edges indicate transport by glacial meltwater, a river, or shoreline processes.

Uses & applications

Used as crushed stone, road aggregate, building stone, and occasionally as a durable decorative or lapidary material. This ordinary cobble has little commercial gemstone value.

Geological facts

Quartzite commonly breaks across grains rather than around them and can scratch ordinary glass. The reddish bands are likely hematite or other iron oxides inherited from the original sandstone or introduced during weathering.

Field identification & locations

Identify it by its very hard, quartz-rich surface, granular texture, and resistance to scratching; a steel knife will not scratch it, while it may scratch glass. Common in metamorphic terrains and glacial or river gravel deposits.